Effects of electrical stimulation of the lateral midbrain tegmentum (LMT) on the copulatory behavior of male rats were examined in our investigation of the role of the LMT in this behavior. Sexually experienced male rats (n = 15) that had been implanted with chronic electrodes under sodium pentobarbital anesthesia were individually tested for copulation with a receptive female under a 30 s on, 30 s off pattern of stimulation. The ejaculation latency, and mount and intromission frequencies decreased significantly during the stimulation tests, indicative of facilitation of copulation. In contrast, a strong stimulus-bound inhibition of copulation was found in 12 males (80%). Six animals who showed accelerated copulation also depressed the lever for self-stimulation at an identical current during the copulation tests. These results suggest that the LMT functions in ejaculation, in the normal expression of intromission and in sexual reward, thereby regulating the copulatory behavior of male rats.