To determine whether female transsexuals have abnormal hypothalamic-pituitary feedback control, pituitary LH and FSH secretory responses to synthetic LRH (100 micrograms iv) were measured in nine female transsexuals with normal menstrual cycles before and after a 7-day course of treatment with diethylstilbestrol (DES; 2 mg/day). Control groups included five heterosexual women and seven heterosexual men. Pituitary responses to LHR in heterosexual women studied in the early follicular phase increased markedly after DES administration and were clearly different from responses in men, which were all inhibited by DES. Responses to LRH in nine transsexual women studied in the early follicular phase differed strikingly from normal women in that gonadotropin responses were not enhanced by DES. The finding that the responses of female transsexuals to DES and LRH were intermediate between the female and the male patterns suggests that a biological abnormality accompanies the psychological abnormality in such patients.