The dose-response characteristics and time-course of amphetamine's effect on motor activity after a single injection given to rats at four different times of the light/dark cycle was investigated using a computerized infrared motor activity recording system. After 7 days of acclimation and 2 days of baseline activity recording, rats received a single subcutaneous injection of vehicle (saline) or 0.6, 1.25 or 10 mg/kg amphetamine at 08.00, 14.00, 20.00 or 02.00. Recording was then resumed for an additional 36 to 48 h. The locomotor indices analyzed were horizontal activity, total distance, vertical activity, stereotypic activity and number of stereotypic movements. All doses (0.6. 1.25 and 10 mg/kg) significantly elevated (P < 0.01) locomotor activity compared to baseline at all times of administration. At all injection times, the maximum increase over baseline generally occurred following the 1.25 mg/kg dose of amphetamine (P < 0.001). The effect of the lower doses (0.6 and 1.25 mg/kg) on forward locomotion remained the same throughout the light/dark cycle regardless of the large difference in baseline motor activity between the light and dark phases. However, the effects of 10 mg/kg amphetamine on general stereotypic behavior, as well as the ability to cause subsequent depression of nocturnal forward ambulation, were dependent on the time of drug administration. These results showed that the circadian rhythms of locomotor and stereotypic effects of amphetamine are different.