Evaluation of the degree of sexual maturation was performed in male mice of two selected lines differing in respect of body weight. Four developmental stages were distinguished: (1) loosening of the skin between the urinary papilla and the anus, (2) projection of the scrotum, (3) descent of the testes to the scrotum, (4) demonstration of sexual drive. Histological examination of the testes revealed that at the first developmental stage the animals had clearly immature gonads. In a few, not numerous, seminiferous tubules spermatogenesis had reached the phase of primary spermatocytes. At the second stage of development the male gonads reached the phase of secondary spermatocytes and sporadically of early spermatids. At the third stage of the animals development, the spermatogenesis observed in numerous cross-sections of the seminiferous tubules reached the spermatid phase and in males at the fourth stage some sperm cells appeared in the lumen of the tubules. A greater diameter of the sperminiferous tubles was demonstrated in the heavy line males than in the light line animals. In males of the heavy line the diameter of the Leydig cells increased between the third and fourth stage. The investigation results suggest that the accepted criteria of determination of sexual maturation stages correspond to various phases of changes taking place in the process of spermatogenesis.