The adrenal glands increase and the thymus and secondary reproductive organs decrease in weight when mice are placed in groups. These changes in weight are related to the size of the population or group and presumably are a reaction to sociopsychological pressures. If such a presumption is correct, reserpine should diminish the differences in the organ weights of grouped and of isolated mice. Grouped and isolated male mice were given 40–50 µg of reserpine (Serpasil, Ciba) per day in their drinking water and the results compared with those from similar numbers of grouped and isolated mice without reserpine. The average number of fights per 10-minute interval per day was 51.4% less in the treated than in the untreated mice for the first 3 days after grouping. Grouped mice, treated and untreated, had heavier adrenals and lighter thymus glands and secondary reproductive organs than their isolated controls. Treatment of grouped mice with reserpine was accompanied by a 5% lower adrenal weight, 28% greater thymus weight, and 12% greater seminal vesicle weight than were found in the untreated grouped mice. Reserpine treatment resulted in apparently comparable changes in the weights of these organs in isolated mice, but the differences were neither as great nor were they significant with the exception of the thymus gland. It was concluded that a) male mice responded to grouping with increases in adrenal weight and decreases in the weights of their thymus glands and secondary reproductive organs whether or not they had received reserpine; b) the response to grouping was not as great in the animals which had received reserpine; c) treatment with reserpine alone effects a reduction of adrenal weight and increases in the weights of the thymus and secondary reproductive organs; and d) reserpine reduces aggressiveness as measured by the number of fights between mice in groups.