The climbing response to apomorphine (AP, 0.075-3.0 mg/kg) and the motor activity response to amphetamine (AMP, 0,3-5.0 mg/kg) were determined in grouped or socially isolated mice. After 4 weeks of differential housing commencing at 5 weeks of age, the individually housed mice showed an increased response only to low doses of these drugs. The responses of the group-housed mice at 5 or 9 weeks of age were identical, ruling out an aging component to the differential responsiveness. Also, the response of separate groups of naive mice to acute treatments of saline, AP (0.15 and 3.0 mg/kg), and AMP (0.3 and 5.0 mg/kg) was examined at 1, 2, 3, and 4 weeks of differential housing. With respect to age-matched group-housed mice, a significantly greater climbing response to AP (0.15 mg/kg) and a significantly greater locomotor activity response to AMP (0.3 mg/kg) were seen after 1 and 3 weeks of individual housing. There were no significant differences in the behavioral responses to the higher doses of AP and AMP.