The reward summation function is the plot of self-stimulation performance as a function of the number of pulses in a train of fixed duration. It has previously been shown that drugs that impair performance compress this curve but do not shift it laterally; whereas when the reinforcing efficacy is reduced by reducing current intensity, the curve shifts laterally. The amount of the shift is a measure of the magnitude of a drug's effect upon reinforcing efficacy. We report here that pimozide shifts the curve to the right in a dose-dependent manner, indicating an impairment of reinforcing efficacy, while amphetamine shifts it to the left, indicating an enhancement of reinforcing efficacy. When the two drugs are given together their effects on the reward summation function cancel out. These results are consistent with the hypothesis that pimozide and amphetamine exert their effects on reinforcing efficacy via one and the same set of dopaminergic synapses.