Sex differences have been observed repeatedly in chronic pain syndromes in both humans and animals, with females showing a higher incidence; it is likely that the gonadal hormones are responsible for these differences. To examine the role of male gonadal hormones on repetitive nociceptive stimulation, we studied male rats, half of them gonadectomized (GDX) and half left intact (INT). Starting from the third week after gonadectomy, they were subjected to the formalin test once a week for 3 weeks (50 micro l formalin 5% injected s.c. in the dorsum of the hind paw: right, left, and right). Formalin-induced licking, flexing, and jerking of the injected paw were recorded and analyzed for each of the three trials. Analysis of variance showed significant differences between GDX and INT animals depending on the trial considered: Trial 1: the GDX and INT groups showed a similar amount of licking, flexing, and paw-jerk; Trials 2 and 3: these responses showed a sort of adaptation in INT animals, not present in the GDX ones, resulting in lower levels of pain responses in INT than GDX. Corticosterone was higher in GDX animals than in INT animals. Testosterone plasma levels were drastically decreased by gonadectomy, whereas estradiol was increased. These data indicate that male gonadal hormones play a key role in inhibiting the behavioral responses to repeated nociceptive stimulation. This suggests that the lower incidence of chronic pain syndromes in males could be caused by the presence of these hormones.