Subplantar injection of 250 micrograms of trypsin in the rat resulted in a biphasic increase in pain sensitivity (hyperalgesia) with peaks at 10 and 150 min separated by a period of decreased sensitivity to pain (hypoalgesia). Hyperalgesia was assessed by a decrease in response latency to a 3.0-kg force applied to the injected hind limb. Response latencies at 150 min were increased in a dose-dependent manner by pretreatment at 90 min with acetaminophen; phenacetin; the arachidonate cyclooxygenase inhibitors aspirin, indomethacin, and ibuprofen; and the opiate analgesics codeine and morphine. ED50s of 17, 13, 10, 0.48, 1.6, 3.9 and 1.2 mg/kg p.o. were obtained for these drugs, respectively. The hyperalgesia present at 150 min was not affected by pretreatment with antiinflammatory steroids, an antihistaminic, an antiserotonin agent, and an anticholinergic. We recommend measurement of drug-induced increase in response latencies produced 150 min after injection of 250 micrograms of trypsin as the basis for a new sensitive and selective analgesic assay. ED50s obtained in this assay correlate well with doses that are used clinically to produce analgesia. Development of the hypoalgesic component was selectively inhibited by pretreatment with an antiserotonin agent. Additional drug studies indicated that the algesic response to the subplantar injection of trypsin is the resultant of independent, temporally overlapping hyperalgesic and hypoalgesic components.