The formalin test: A quantitative study of the analgesic effects of morphine, meperidine, and brain stem stimulation in rats and cats

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