A method for assessing pain and analgesia in rats and cats is described. The procedure involves subcutaneous injection of dilute formalin into the forepaw, after which the animal's responses are rated according to objective behavioral criteria. The formalin test is a statistically valid technique which has two advantages over other pain tests: (1) little or no restraint is necessary, permitting unhindered observation of the complete range of behavioral responses; and (2) the pain stimulus is continuous rather than transient, thus bearing greater resemblance to most clinical pain. The analgesic effects of morphine, meperidine, and stimulation of the periaqueductal grey matter are evaluated using this test.