A hand-made apparatus which is easily manipulative, cheap, durable and clean was used to measure ambulatory activity of a mouse. The apparatus was assembled with the same type of two plastic round basins commercially available (25 cm in diameter) put one upon another. A pivot was fixed at the center of the outsurface of the inner basin (A), and its open end was inserted into the open socket fixed at the center on the bottom of the outer basin (B). Three microswitches equipped on the brim of (B) at equal intervals were activated and the counts were recorded according to the tiltings of (A) through the movements of the mouse. In order to examine accuracy of the measurement, effects of d-amphetamine (1.25 approximately 5.0 mg/kg), methamphetamine (1.0 approximately 4.0 mg/kg), cocaine (10 approximately 40 mg/kg) and morphine (5.0 approximately 20.0 mg/kg) on the ambulatory activity were investigated. Marked accelerating effects were observed dose dependently after the administrations of all the drugs. Furthermore, the patterns of the activity showed characteristic properties of each drug. This method is especially useful to measure the acute effect of drugs on the ambulatory activity in mice, and many units can be set up at the same time.