A six-point, incomplete block assay of sublingual buprenorphine and intramuscular morphine has been carried out, providing valid relative potency estimates of the two drugs in terms of total relief on both categorical and visual analog scales. Sublingual buprenorphine was about 15.5 times as potent as intramuscular morphine in terms of these total relief estimates. Similar relative potency estimates were obtained using first-dose-only data. There was no evidence of interaction by day in the crossover data, and the crossover study proved more efficient and provided tighter confidence limits. Sublingual buprenorphine produced a lower peak effect than intramuscular morphine. At equivalent peak effects, it produced longer-lasting analgesia. Side effect occurrence was roughly comparable for the two drugs, and no evidence of narcotic antagonist activity was seen after buprenorphine. The six-point assay proved to be effective in defining the dose-effect curves and relative potencies of the two drugs.