Hypothalamic lesions of serotoninergic afferents following bilateral stereotoxic injections of the neurotoxin 5,7-dihydroxytryptamine (5,7-DHT) into the region of the ventromedial hypothalamus enhanced the effects of hormonally induced lordosis responding in both male and female rats. However, similarities and differences in the enhanced responsiveness to ovarian hormones were observed between the sexes. Compared with sham-lesioned controls, lesioned males displayed elevated lordosis responding to estradiol (E) priming alone, as well as to priming with E followed by progesterone (P). On the other hand, lesioned females displayed elevated lordosis in response to E priming alone, but were not different from controls in a synergistic facilitation of lordosis by P after E priming. With respect to receptivity, neither lesioned nor control males displayed ear-wiggling and hop-darting in response to E + P, whereas both lesioned and control females were proceptive following this treatment. Therefore, hypothalamic lesions following 5,7-DHT increase lordosis, but fail to unmask in males the responsiveness to E + P priming mediating proceptive behaviours by females. Further, the levels of lordosis responding displayed by lesioned males are lower than those of either lesioned or control females after E + P priming, as well as those of lesioned females after E priming alone, thus indicating that other inhibitory mechanisms continue to operate in the lesioned males.