Male rats were tested with females continuously present and with the presence of the female after each sex act contingent upon a bar press (Experiment 1). The sole effect of the bar-press requirement was to increase the intervals between copulations before ejaculation. In Experiment 2, intermittent shocks were superimposed upon the conditions of Experiment 1. Shocks were followed with short latency by mounts in the ad-lib condition and by bar presses and mounts in the operant condition. The pacing of copulatory acts before ejaculation is inferred to result from an interaction of stimuli from the female and feedback from copulatory acts; after ejaculation, factors primarily endogenous to the male govern the timing of resumption of copulation. Although a great deal is known of the neural, endocrine, stimulus, and experiential factors affecting the probability that a male rat will initiate a copulatory sequence, relatively little is known of the factors that control the temporal patterning of events within a copulatory sequence. Sources of stimulation that may contribute to temporal regulation include (a) the surface of the penis; (6) deeper pressure receptors in the penis; (c) feedback from other areas involved in mounting, e.g., feet, legs, hips, etc.; (d) the receptive female's odor and behavior; and (e) endogenous fluctuations in sexual arousal. Previous research has indicated that at least for experienced male rats, surface anesthetics applied to the penis prevent the male from gaining intromissions but do not interfere with the motivation for copulation (Adler & Bermant, 1966; Carlsson & Larsson, 1964) or with the temporal patterning of bouts of mounting (Sachs & Barfield, 1970). Pressure receptors in the penis have been implicated by Hart's (1972) demonstration that spinal males with surface-anesthetized penes respond normally to continuously maintained pressure upon the penis.