Production of testosterone by Leydig cells during the postnatal ontogeny in pubescence under in vitro stimulation by chorionic gonadotropin, dibutiryl-cAMP, and pregnenolon was studied in males of four inbred mouse lines (BALB/c, RT, CBA/Lac, and A/He) and their F1 reciprocal hybrids. Highly statistically significant association between the animal genotype and age was revealed for all parameters studied, which indicates the genotype-dependent formation of the Leydig cell hormone function during the postnatal ontogeny. The effect of genotype was characterized by two specific features. First, in each postnatal ontogeny stages examined correlative genetic variability in respect of the cAMP- and substrate-dependent indices of Leydig cell reactivity was observed. Second, during postnatal ontogeny coordinated genetic variability was subjected to substantial ontogenetic rearrangements. Definite pattern of genetic differences in the Leydig cell hormone activity was formed only at the late pubertal--early post- pubertal stage (60th day after birth). This process coincided with the completion of the Leydig cell morphological differentiation and the appearance of mature cells in the population. Thus, formation of the Leydig cell hormone activity during postnatal ontogeny is under coordinated genetic control, which is also subjected to substantial changes during pubertal differentiation.