The effects of intracerebroventricular (i.c.v.) administrations of the progesterone metabolite, 3 alpha-hydroxy-5 alpha-pregnan-20-one (3A5P), on the nociceptive responses of male mice were examined. 3A5P elicited significant, dose-dependent (0.001-1.0 microgram) analgesia for 90-120 min after administration. These effects of 3A5P were significantly more potent than those of progesterone. The stereoisomer, 3 beta-hydroxy-5 alpha-pregnan-20 one (3B5P), failed to affect the nociceptive responses, indicating that the analgesic effect of 3A5P is stereospecific. The analgesic effects of 3A5P were blocked by peripheral administrations of the GABA antagonists, bicuculline and picrotoxin, and reduced by both the opiate and benzodiazepine antagonists, naloxone and Ro 15-788, respectively. The calcium channel antagonists, nifedipine and verapamil, enhanced 3A5P-induced analgesia but had no evident effects on the actions of 3B5P. These results suggest that the central analgesic effects of the progesterone metabolite, 3A5P, may arise via mechanisms involving calcium channels, the GABA-benzodiazepine-chloride complex and endogenous opioid systems.