In an established social group of rhesus monkeys (macaca mulatta) consisting of 1 adult male, 6 adult females, 1 adolescent male and 2 infants, 3 subjects were selected to study the effects of intramuscular methaqualone (10 mg/kg) on behavioral changes in the treated animals, as well as the rest of the colony. Three drug trials alternating with saline controls given to the dominant male, a mid-ranking female and the adolescent male. 20 h of observations were made prior to treatment to serve as a baseline, using 40 discrete behaviors. For each drug and saline period, 2 h of continuous observation was directed towards each target animal (total 36 h), and 12-min behavioral samplings were carried out on each untreated subject in the colony (36 h). Dependent upon the sex and social rank of the subject, different behavioral changes occurred during drug treatment, which were consistent from trial to trial. New behaviors were observed, such as active grooming, autofellatio and masturbation in both males, while some previous behaviors disappeared or decreased. The mid-ranking female showed an increase in aggressive behavior. A biphasic action of the drug was also observed in all subjects. During the first 80--100 min, the males were mainly passive or engaged in grooming, but after this primary phase, they started either to masturbate or autofellate. After approximately 2 h, the social status and behavior of the animals returned to predrug levels. During the drug treatment, marked changes occurred in the behavior of other colony members, e.g. when the dominant male's vigilance was lowered, the highest ranking female replaced him in rank. In general, affiliative activities were increased under the influence of methaqualone, manifesting themselves mainly in grooming, hudding or sitting together: phenomena somewhat parallel to those described among humans taking the drug in group settings.