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A method is described for evaluating antinociceptive activity in rats using writhing responses induced by 4% sodium chloride solution. Narcotic and nonnarcotic analgesics, narcotic antagonists except naloxone, and antipyretic and nonsteroidal antiinflammatory drugs were effectively evaluated at relatively low doses using this method. Parenteral ED50 values of these compounds ran parallel to their clinical doses. The method detected not only the analgesic action of the above drugs but also that of methamphetamine. In spite of its high sensitivity, the method was specific in the sense that CNS-depressant, muscle relaxant, anticholinergic, and antihistamine drugs and anesthetics were inactive even at toxic doses. On the other hand, the decrease in the analgesic activity of morphine upon chronic injection was also observed with this method. The writhing test with 4% sodium chloride in rats was concluded to be a sensitive and specific test for analgesic drugs and useful for examining changes in analgesic activity of drugs during chronic administration.