In this study, morphine-3-glucuronide (M3G), the major plasma and urinary metabolite of morphine, was shown to be a potent antagonist of morphine analgesia when administered to rats by the intra-cerebroventricular (i.c.v.) route. The antagonism of morphine analgesia was observed irrespective of whether i.c.v. M3G (2.5 or 3.0 micrograms) was administered 15 mins prior to or 15 mins after i.c.v. morphine (20 micrograms). When M3G (10mg) was administered intraperitoneally (i.p.) to rats 30-40 mins prior to morphine (1.5mg i.p.), the analgesic response was significantly reduced compared to administration of morphine (1.5mg i.p.) alone. It was further demonstrated that i.c.v. M3G (2.0 micrograms) antagonized the analgesic effects of subsequently administered i.c.v. morphine-6-glucuronide (0.25 micrograms).