The hypothesis that allogrooming functions as a tension-reduction mechanism was tested. Tension was measured by the frequency of displacement activities by an animal. Two adult male and 11 adult female Java (or long-tailed) monkeys (Macaca fascicularis) that were relatively unfamiliar with each other were paired 1 hour per day for five consecutive days during the periovulatory portion of the menstrual cycle. Female allogrooming was found to reduce the frequency of male displacement activities both during the course of interaction and outside it, and this decrease was proportional to the amount of allogrooming received. Female allogrooming did not, however, exert long-lasting effects on the frequency of female displacement activities. An increase in the frequency of male displacement activities was recorded during the 10-second interval immediately after the end of the female allogrooming bouts. Neither postinhibitory rebound nor frustration owing to the cessation of a pleasant situation, i.e., the two advanced explanatory hypotheses, accounted for this increase. The results of the present study concur with physiological findings that support the tension-reduction hypothesis. The social function of allogrooming appears quite important and is entirely compatible with the functional hypothesis that emphasizes hygiene.