In an earlier article an enlarged subpopulation of vasopressin containing neurons was found in the suprachiasmatic nucleus (SCN) of homosexual men as compared to heterosexuals. The present study investigates the possibility that the number of vasopressin neurons in the SCN and sexual partner preference behavior in male rats are both influenced by sex hormones during brain development. For this purpose, we studied groups of adult male rats that had been treated either prenatally or pre- and postnatally with the aromatase inhibitor ATD (1,4,6-androstatriene-3,17-dione) which blocks the aromatization of testosterone to estradiol. Rats treated with ATD in both pre- and postnatal periods showed 'bisexual' partner preference behavior and appeared to have 59% more vasopressin-expressing neurons in the SCN than the controls. The prenatally treated rats did not differ from the controls. This observation supports the hypothesis that the increased number of vasopressin neurons found earlier in the SCN of adult homosexual men might reflect differences that took place in the interaction between sex hormones and the brain early in development.