The experiment was conducted on 395 ewes and their 585 half-brothers divided into groups according to sires (50 and 37 groups, respectively). The date of the first oestrus in yearling ewes, date of successful mating of older ewes and the number of lambs born were studied. Testicular diameter and scrotum circumference were measured in growing rams during stimulatory and inhibitory photoperiods. The results of testicular measurements of these young rams were compared with the corresponding periods of sexual activity and reproductivity of related ewes. It was found that the sire's genotype affected the timing of sexual activity of ewes during the spring-summer breeding season. The date of successful mating of yearling ewes was highly correlated (P≤0.01) with their body mass. The correlation between the testis diameter index of rams at 28 days of age and the period of the first oestrus in yearling ewes was significant (P≤0.05; r = 0.35), while that between this index and the date of successful mating during the first breeding season was highly significant (P≤0.01; r = 0.37). Significant correlations were also found between the scrotum circumference index at 100 days of age and the period when fertilization of ewes occurred during the first breeding season (r = 0.28) and between this index and the number of lambs born in the first lambing (r = -0.27).