Social hierarchy in the animal community is a structure of behavior relations when members of a social group vary in their ability to acquire resources in the presence of others. The aim of our study was to investigate social dominance effects on development of the reproduction function in male mice. Sperm quality, relative testis and epididymis weights were evaluated in male mice of PT and CBA/Lac inbred strains at 70 day of age after pairing throughout the 32 day period. The effect of social dominance on sperm quality was observed only in males of the CBA/Lac strain. The dominant males had higher sperm count than subordinates or control. The result revealed also higher percent of abnormal sperm among dominant or subordinate males compared with control. It was suggested that the effects of social competition on spermatogenesis depend on a genotype and social status.