U-50,488H has been shown to be a naloxone antagonizable analgesic in rodents. However, the dose of naloxone needed for antagonism is higher than it is for morphine. U-50,488H does not produce physical dependence; however it does produce tolerance upon chronic administration. U-50,488H is cross tolerant with bremazocine but not with morphine. Monkeys trained to discriminate ethylketocyclazocine (EKC) from saline show a complete generalization to U-50,488H but not to morphine. The evaluation of U-50,488H in 3H-EKC site-selective binding indicated that U-50,488H has a high affinity for the kappa receptor (Ki = 114 nM) and a low affinity for the mu receptor (Ki = 6100 nM). The ratio of Ku/Kk was 0.08 for morphine, 0.4 for dynorphin, and 53.5 for U-50,488H. The data suggest that U-50,488H is a selective agonist at the opioid kappa receptor.