In castrated male rats, a radiofrequency lesion was made in the dorsal raphe nucleus (DRL) and lordosis behavior was observed following treatment with estrogen. After the behavioral test, brains were removed and the contents of 5-HT and 5-HIAA in the forebrain were measured by high pressure liquid chromatography (HPLC). In the results, only 2 of 16 control males without brain surgery showed lordosis, and the mean lordosis quotient (LQ) was extremely low when compared to that in control females. In contrast, all male rats with DRL displayed lordosis and the mean LQ was higher than that of control males without brain surgery but lower than that in control females (P < 0.001). In the DRL males, 5-HT and 5-HIAA contents in the septum (SPT), the preoptic area (POA), the ventromedial hypothalamus (VMH) and the striatum (STM) were lower than those in control male and female groups (P < 0.001). These results suggest that the dorsal raphe nucleus prevents male rats from showing lordosis by serotonergic influence in the forebrain. In addition, HPLC results showed that levels of the 5-HT in the SPT, the POA and the VMH in the female group were higher than those in the control male group (P < 0.05). In female rats, the POA (P < 0.01) and the VMH (P < 0.05) contained larger 5-HT than those in the SPT and the STM, but there were no difference of 5-HT contents in the male rat.