Male tree shrews (46 animals in all) were kept under distinct social conditions. Two kinds of controls were investigated: isolated animals as well as males out of female/male couples. Comparing these two groups only little isolation effect was observed. With the other individuals, initially isolated, dyads were formed which coexisted for 7 or 50 days. In a third group of dyads, after 50 days of coexistence social circumstances were aggravated for 5 more days by providing only half of the cage. All dyads established clear cut dominance relationship. Animals behaving dominantly developed no signs of gonadal regression rather that of stimulation concerning especially with the epididymis. In contrast, animals behaving as subordinates revealed profound regressive changes although differentiated with respect to any parameter, both morphological and conditional. In testis, chronical subordination (50 days, mainly 50 + 5 days) leads to a loss of organ weight, narrowing of seminiferous tubules and to enlarged spermatogonia nuclei. The case is similar in the epididymis: loss of organ weight, lowered epithelium and an enormously increased frequency of immature germ cells within tail segments. Whereas prostate weight drops continuously, striking histological regression occurs already after 7 days (extreme diminution of epithelial height and of epithelial cell nuclei).