Previously isolated, adult male mice of a wild-derived strain were sham-operated or pinealectomized (pinx) and 14 days later were paired for 15 min, then grouped three per cage for 7 days. During 15 min pairings, mean latency to initiation of fighting was increased twofold and duration of fighting was reduced 35 to 41 percent in pairs comprised of one or both pinx animals. Although pinealectomy failed to influence which pair member initiated fighting, sham males were ranked as dominant in 75 percent of sham-pinx pairs. Pinealectomy reduced adrenal gland weights among isolated animals and increased in general the effects of social subordination in grouped males. Compared to sham subordinates, pinx subordinates experienced adrenal enlargment and twofold greater wounding and had lighter thymus glands and final body weights. Additionally, however, pinealectomy prevented atrophy of seminal vesicles among subordinates. Thus, present results suggest a role of the pineal gland in influencing intermale aggression and linking social stimuli with testis dysfunction in adult house mice.