The frequency with which one monkey opened a door for two minutes of visual and physical contact with another monkey was examined as a function of several experimental procedures. In the first series of manipulations removal of the second monkey allowed a comparison between the effects of the social reinforcing object and the effects of the non-social stimuli incidental to delivery of the social reinforcer. During daily four-hour sessions, reinforcement rates (number of door openings/unit time) with a monkey in the adjacent cage averaged several times those when no monkey was in the adjacent cage. Removal of the second monkey did not significantly affect rates of food and water reinforcement, but removal of food and water contingencies did increase rates of door opening to an empty cage. Idiosyncratic factors contributed to above-zero rates when the cage was empty. For example, one subject's empty-cage rates were halved, and his amount of stereotyped rocking nearly stopped, by a wall placed next to the door. The existence of pertinent variables peculiar to the individual subject and to the particular apparatus directs attention to the necessity of control procedures. In the second set of conditions two baboons lived in the cages 24 hours a day and were under continuous illumination, as they had been for several months. During a short initial period when either monkey could open the door, and in a second longer period when only one could open the door, the monkeys had a very regular day length of approximately 14 hours, which remained synchronous with clock time. The experiment did not identify the pertinent elements entraining the monkeys' activities. This persisting alignment with clock time under reasonably stable conditions suggests that the entraining environmental stimuli were more subtle than those demonstrated in the literature. The monkeys also revealed a distinctive patterning of frequency of social contact as a function of time of day. The distribution had a midmorning peak, a midday low, and a minor afternoon peak, the same pattern recently documented as occurring in troops of wild baboons. This pattern was only barely noticeable in distributions from individual days, and became significant only when averaged across days. To a lesser degree, similar patterns were evident in distributions of food and water reinforcement rates. In the final set of procedures two levels of food deprivation, two levels of social deprivation, and two times of day were produced by alternating the subjects in morning and afternoon sessions, by conducting only morning or only afternoon sessions, and by pre-feeding and "pre-socializing" in a sequence designed to contrast the effect of one condition against another. Regardless of deprivation of social or other reinforcers, the subjects displayed a higher rate of social reinforcement in the mornings than in the afternoons. An early morning "pre-socializing" session slightly lowered the usual morning rate of social reinforcement, but preceding an afternoon session with a morning session, an early morning and a morning session, or by no session since the previous afternoon, had no effect on the afternoon social reinforcement rates. Independence of social reinforcement rates from variations in food deprivation, and low frequencies of eating with the door open implied that the presence of food did not appreciably affect rates of social reinforcement. These studies established social reinforcement under controlled laboratory conditions as a strong reinforcer capable of maintaining behavior over long periods of time, and elucidated a pertinent variable in social reinforcement, that of diurnal rhythms.