The pharmacological effects of morphine, namely analgesic, hyperthermic and cataleptic effects, were assessed in rats rendered tolerant to U-50,488H, a kappa opioid receptor agonist. Male Sprague-Dawley rats were injected intraperitoneally with U-50,488H (25 mg/kg) twice a day for four days. The rats which served as controls were injected similarly with the vehicle. Chronic administration of U-50,488H resulted in the development of tolerance to its analgesic and hypothermic effects, but not to its diuretic effect. The development of tolerance to the pharmacological effects of U-50,488H was associated with decreased binding of [3H]ethylketocyclazocine [( 3H]EKC) to brain and spinal cord membranes. The decreased binding of [3H]EKC in U-50,488H-treated rats was due to changes in the Bmax value; the Kd values remained unaltered. Intraperitoneal administration of morphine (8 mg/kg) to rats produced analgesia (as determined by the tail-flick test) and hyperthermia. A dose of 50 mg/kg of morphine produced cataleptic response. The intensity of analgesic, hyperthermic and cataleptic effects of morphine were unaltered in rats tolerant to U-50,488H. The development of tolerance to analgesic and hypothermic effects of U-50,488H were associated with down-regulation of brain and spinal cord kappa opioid receptors. Finally, U-50,488H does not confer cross-tolerance to morphine, a predominantly mu opioid receptor agonist.