Marked depression of locomotor activity was observed in writhing mice given acetic acid i.p. This depression of the activity was evidenced by a squatting posture in reaction to pain. The hypoactivity was reversed dose-dependently by nonnarcotic analgesics such as acetyl-salicylic acid, aminopyrine and mefenamic acid in smaller dosages than those obtained by the conventional writhing syndrome test, and was also reversed dose-dependently by narcotic analgesics such as morphine, pethidine and codeine. Consequently, this hypoactivity test proved to be useful for analgesics screening.