This study was designed to determine the magnitude and duration of the analgesic effect of three commonly used opioids: buprenorphine (0.5 mg/kg for rats; 2.0 mg/kg for mice), butorphanol (2.0 mg/kg for rats; 5.0 mg/kg for mice), and morphine (10 mg/kg for rats and mice). We used two standard tests, the hot plate and tail flick assays, to measure opioid analgesia in 62 male, 200 to 300 g Sprague-Dawley rats and 61 male, 25 to 35 g ICR mice. We obtained five baseline measurements then administered the drugs subcutaneously. Morphine gave the highest analgesic effect and was intermediate in duration (2 to 3 h in rats and mice) of analgesia. Butorphanol provided the lowest level of and shortest (1 to 2 h in rats and mice) analgesia. Buprenorphine had an intermediate analgesic effect and the longest duration (6 to 8 h in rats and 3 to 5 h in mice). In light of our results, we recommend the use of morphine (with frequent redosing) for severe pain, butorphanol for mild pain of short duration, and buprenorphine for mild to moderate pain of increased duration. The dosing intervals suggested by our study are 2 to 3 h for morphine in both rats and mice, 1 to 2 h for butorphanol in both rats and mice; and 6 to 8 h in rats and 3 to 5 h in mice for buprenorphine.