Morphine, morphine-6-glucuronide (M6G) tolerance and cross-tolerance between morphine and M6G have been evaluated in mice. Daily administration of equipotent doses of M6G and morphine induced similar declines in antinociception over 9 days of treatment. However, a higher dose of M6G than morphine is required in tolerant animals to recover the initial response. In studies where daily morphine doses were substituted by M6G administration, on specific days, there was a significant fall in M6G antinociception on those days immediately following morphine administration, relative to the response to continued morphine (a decrease of 53.7% on day 2, P < 0.001 and a decrease of 62.5% on day 11, P < 0.05) and M6G (a decrease of 45.4% on day 2, P < 0.05) exposure. The decrease was independent of treatment duration and dosage. This decrease in the antinociceptive effect of M6G after morphine was avoided after clofibrate treatment, an inhibitor of (-)morphine metabolism. Determination of morphine and its metabolites in plasma revealed that morphine-3-glucuronide (M3G) concentration was significantly lower (P < 0.001) in animals treated with clofibrate (8.3 +/- 8.3 ng/ml) than in controls (422 +/- 80 ng/ml). The dose-response curve for M6G was shifted to the right by prior administration of M3G. These results suggest that during morphine treatment the antinociceptive effect of M6G may be antagonized by the other metabolite, M3G.