RESULTS AND DISCUSSION. Taking into account the marked hares as a whole, we find that the value of the median survival rate was 124 days, independently of any other variable. The maximum survival rates were 376 days corresponding to an adult male. 10% of radiotracked Iberian hares lived more than 250 days. If we do the separation of the marked animals by sex, the median values obtained were 131 days for males and 116 days for females. With regard to survival evolution of radio tracked hares released, we observed that females had more capacity of survival than males during the days following the release (Graphic 1). None of the females was able to live 245 days after their release. If we do the separation of the marked animals by age, the median values obtained were 121 days for the sub-adult ones and 127 for the adult ones. In the same way, if we study the influence of sex and age altogether, over the median values of the survival rates, we obtain that the median survival rate of sub-adult males was 100 days, while for adult males that value raises to 162 days. In the case of females, we obtain a median survival rate of 142 days for the sub-adult hares, while for the adult ones the value was reduced to 84 days (Graphic 2). In the statistic treatment of the data, we did not find significant differences due to sex nor age of the marked animals. With regard to causes of death, of the total of drops registered (Graphic 3), we assigned 32% to predation by carnivorous, fox mainly and sheepdog (Photo 4); 21% to doubtful causes of death; 16% to several diseases (Photo 1): European brown hare syndrome, pseudotuberculosis, etc.; 16% due to defect in the design of the necklace radio transmitter (Photo 3), and finally 15% to road traffic accident (Photo 2). SURVIVAL OF RADIOTRACKED IBERIAN HARES (Lepus granatensis) IN THE CASTILIAN PLAIN. BARTOLOMÉ, D.J.; PÉREZ, J.A.; DÍEZ, C.; OLMEDO, J.A.; ALONSO, M.E. Dpt. Animal Production II, Faculty of Veterinary of Leon Spain. E.mail: XXVI INTERNATIONAL IUGB (International Union of Game Biologists) BRAGA, INTRODUCTION AND OBJECTIVES. Iberian hare is one of the most desired cynegetic species by hunters. Due to its excellent adaptability, its area of distribution is very large, which extends through all the Iberian Peninsula to the south of the river Ebro. It is paradoxical that an animal with so much tradition and importance in the cynegetic Spanish activities has not been the target of a larger number of field studies in our country. In view of the exigency of administration of the cynegetic resources, through the Plans of Cynegetic Organization, it is urgent the realization of studies of populations and species-environment relation, over Iberian hares. Such an urgency is bigger in the case of Iberian hare (Lepus granatensis), which, without a doubt, is the least studied species within the Spanish small game. The main objective of this work was to evaluate the survival of autochthonous Iberian hares in a natural environment that they know perfectly, and to determine the most frequent causes of mortality of the Iberian hares in the Castilian plain. MATERIAL AND METHODS. 1. STUDY AREA. The realization of the study was carried out in the farm Coto Bajo of Matallana, property of the Most Excellent Country Council of Valladolid, located in the district of Villalba de los Alcores, in the North extreme of the province of Valladolid (41º 53’ 45”-41º 55’ 15” N latitude and 4º 51’ 10”-4º 52’ 50” W longitude), with a total extension of approximately 308 has. where a cynegetic fence has been built around. The farm lacks cynegetic exploitation. Therefore, the density of the different species is only conditioned by the relation predator-prey. With regard to permanent vegetation, it must be highlighted the important amount of reforested plants that includes several species such as pines, brushes and clump of brambles. As regards farming, they stand out lands of cereal, concretely barley and alfalfa. 2. ANIMALS. With regard to the animals, a total of 22 hares were captured, all of them in the farm, therefore autochthonous. Among all the 22 animals, only 19 were used for the present study, the other 3 were cast off due to several causes. As regards the age of the hares, 9 sub-adults animals and 10 adult ones were used. The sex proportion was 9 females and 10 males. All the captures were done at night. Each hare was localized with the help of a jeep and a hand spotlight. Later, the animal was guided to the place where a net of nylon had been placed the afternoon before. Once there, the animal was captured and the corresponding radio-transmitter was fixed as a collar to its neck. After being determined the sex and approximate age of the animal, and after identifying it by means of a numbered metallic crotal placed in one of its ears, the animal was immediately liberated. Finally, a weekly and individualized study of the animals was realized by means of the radio-tracking system, already used with hares by others authors such as RICCI in 1983; RODRIGUEZ et al. in 1997 and ANGELICI et al. in Each localization stopped at the moment of detecting clear activity. Finally, each member had its own file card where it was noted in a map of the farm (scale 1:10.000) the localization of the animal. In the case of finding a dead hare, it was taken a photograph and the body was taken in order to determine the cause of its death by means of a necropsy. 3. DATA PROCESSING. To carry out the statistical study was used the computer program of statistical SPSs© (version 10.0) for WINDOWS©. BIBLIOGRAPHY. ANGELICI, F.M; RIGA, F.; BOITANI, L.; LUISELLI, L. (1999). Use of dens by radiotracked brown hares Lepus europaeus. Behavioural Processes, 47: BROEKHUIZEN, S.; MAASKAMP, F. (1982). Movements, home range and clustering in the European hare Lepus europaeus in the Netherlands. Z. Saugetierkunde, 47: (2001). RICCI, J.C. (1983). Study of released hares (Lepus europaeus) by radio- telemetry: mortality, dispersion and space use. Acta ecol., 4: RODRÍGUEZ, M.; PALACIOS, J.; MARTÍN, J. A. (1997). La liebre. Ediciones Mundiprensa. Barcelona. 160 pp. Graphic 3. Causes of death to radio-tracked iberian hares. Graphic 2. Survival rates of radiotracked iberian hares. Photo 2. Death by road traffic accident. Photo 1. Death by disease. Photo 3. Death by defect in the design of the necklace radiotransmitter. Photo 4. Death by fox predation. Graphic 1. Evolution rates of survival of radio controlled iberian hares by sex and age.