1. INTRODUCTION DISCUSSION EMOTIONAL RESPONSE TO EROTIC STIMULI: GENDER DIFFERENCES According to Lang’s model, emotional response is organised along the two main dimensions of valence and arousal, considered as primitive motivation parameters in subcortical brain areas. In particular, skin conductance is directly related to stimulus emotional intensity. It is a parameter not under conscience control, representing a pure visceral reaction. Literature reports an increase of this psychophysiological parameter looking at nudes images of both sexes. The aim of this study was to evaluate the emotional physiological response to erotic visual stimuli in a sample of MtF and FtM transsexuals compared with a control group. SUBJECTS Twenty-four transsexuals (12 MtF and 12 FtM), some of them in hormonal therapy, participated to the study. Twelve heterossexual women and 12 heterossexual men, matched for age and educational level formed the control group. MATERIALS Sixty coloured slide were chosen from the International Affective Picture System, I.A.P.S. (Lang, 1999) for their affective impact (valence, arousal and dominance). These pictures were spilt in three set: twenty positive, twenty negative and twenty neutral slides. Eight of the pleasant pictures represented erotic stimuli: four female and four male nudes. Each image was presented for a period of 6 seconds, followed by a blank screen for other six seconds. The slides were presented using a run by Windows NT workstation. During the experimental session skin conductance was recorded with a multichannel polygraph Galileo Sirius Star Ebineuro. Data were analysed using the t-test for paired samples. A p value <.05 was considered statistically significant. The present study is the first one evaluating autonomic nervous system (ANS) parameters (skin conductance) in response to erotic visual stimuli in transsexuals. Our results evidences a coherence between sexual identity and sexual orientation in transsexuals. In fact, they feel to belong to the opposite sex, so they should experience a sexual attraction toward people of their own sex. Literature shows an evidence for male-female differences in brain structures. Transsexuals experience themselves as being of the opposite sex, despite having their biological characteristics. Some studies show a female brain structure in genetically male transsexual and support the hypothesis that gender identity develops as a result of an interaction between the developing brain and sex hormones. Recent brain research has revealed structural differences in the hypothalamus in relation to biological sex and sexual orientation. The bed nucleus of the stria terminal ( BNST) might participate in forming gender- identity. A study by Swaab revealed that BSNT, in the transsexual MtF has the same dimension as those observed in the women. Our study gives a contribution to the scientific knowledge of the biological base of the transsexualism showing a further similar ANS pattern between MtF and F on one side and FtM and M on the other. REFERENCES Lang, P.J., Bradley, M.M., Cuthbert, B.N. International affective picture system (IAPS): Instruction manual and affective ratings. Technical Report A-4, 1999 The Center For Research in Psychophysiology, University of Florida. Kruijver FP, Zhou JN, Pool CW et all. Male-to-female transsexuals have female neuron numbers in a limbic nucleus. J Clin Endocrinol Metab.2000 May; 85 (5) : 2034-41 Zhous JN, hofman MA, Gooren LJ et all. A sex differences in the human brain and its relation to transsexuality Nature. 1995 Nov 2; 378 (6552): 68-70 Swaab DF, Gooren LJ, Hofman MA. J Homosex. 1995; 28 ( 3-4): 284-301 Brain research, gender and sexual orientation. Molo MT, Perozzo P, Cappai E, Castelli L., Vighetti S, Sandrone L, Cantafio P, Sassu O, Latini Corazzini L CE.R.NE, TURIN, ITALY. CONTACT: mtmolo@attglobal. net WEB SITE: www.cerne.it WEB SITE: www.cerne.it MALE and FtM EVIDENCED A SIMILAR PATTERN OF ACTIVATION; BOTH THE GROUPS SHOWED A MAJOR AROUSAL LOOKING AT FEMALE NUDES. FEMALE AND MtF EVIDENCED ALSO A SIMILAR PATTERN OF ACTIVATION THEY SHOWED A MAJOR AROUSAL LOOKING AT MALE NUDES. METHODS RESULTS * p<.05