We had shown that the mechanism of analgesia induced by swimming stress in Wistar males is closely related to stress, some of the relation to this mechanism with cortical hormones. The aims of this research is to study the sexual differences in the mechanism of analgesia induced by swimming stress in both male and female Wistar rats and the possibility of its relation with sexual hormones. The male and female show an increase in the threshold of pain as result of swimming stress (Swimming in the water during 3 minutes, with 16 C). The analgesia in the females was more important compared with males. The pretreatment with naloxone led to a partial inhibition of stress- induced analgesia in both sexes, but in males it was more important.