The effects of acute IP administration of phencyclidine (PCP), d-amphetamine (AMPH) and pentobarbital (PB) were determined in 10 mice trained to lever press on a differential reinforcement of low rate 10 sec schedule of sweetened milk presentation. The effects of PCP were highly consistent, with large response rate increases (and a corresponding shift toward shorter interresponse times) at doses of 1 and 3 mg/kg. Higher doses generally decreased response rates and resulted in a bimodal interresponse time distribution. The effects of AMPH were similar to PCP but less consistent. Although some of the subjects showed substantial response rate increases at doses between 0.3 and 10 mg/kg, half of the subjects did not show increased response rates at any dose. The effects of AMPH on the interresponse time distribution were similar to PCP. The effects of PB were least like those of PCP. The effect in most subjects was to produce a dose-related decrease in response rate and a flattening of the interresponse time distribution. Occasional small response rate increases were observed with PB.