Relationship between neonatal testosterone and 5-hydroxytryptamine (5-HT) in controlling the pattern of LH release in adult rats was investigated in this study. Hypothalamic 5-HT concentrations are transiently lower in male as compared to female Wistar rats in the second week post-partum (pp). It has also been shown that pharmacologically potentiating 5-HT activity during this period feminizes certain aspects of sexually differentiated behaviors in adult males and androgenized females. In order to investigate whether neonatal testosterone and 5-HT interact to influence physiological and morphological brain sexual differences, intact females androgenized females and intact males were treated with the 5HT2 against (-)[2,5 dimethoxy-4-iodophenyl]-2- amino propane HCl [(-)] (DOI), over days 8-16 pp in androgenized females (250 mg testosterone propionate (TP), day 2 pp). DOI prevented the delay in vaginal opening, but did not prevent the androgen-induced constant oestrous in females treated with 100 ng TP at day 2 pp. DOI overcome the neonatal androgen effect in suppressing the positive feedback of ovarian steroids in a few males and androgenized females. DOI had a feminizing effect on the volume of the anteroventral periventricular nucleus (normally smaller in males, by significantly increasing its volume in male and androgenized females. It also had a significant antagonistic effect on the testosterone-induced increase in the volume of the sexually dimorphic nucleus of the preoptic area in males and androgenized females. These findings support the evidence that raised 5-HT activity in the second week of life antagonizes the masculinizng effect of neonatal testosterone.