Purpose. To study the influence of the weight of testes during the prefunctional period on the morpho-physiological and reproductive performance of roosters. Methods. Ultrasound determination of the size of testes in roosters; zootechnical and statistical. Results. Ultrasound diagnostics was first used to determine the size of testes in roosters, which allowed to divide roosters non-invasively into groups according to the sizes of testes and to trace their changes. Compared to M-roosters (with the relative weight of testes of over 40 and up to 40 mg / 100 g, respectively), four-month-old B-roosters of Silver Leghorn demonstrated significantly higher average values of relative weight of the testes (4.2 times), the quality of sperm (concentration and mobility – 1.5 and 1.3 times higher), as well as relative weight of crest (1.4 times higher). Four-month-old progeny of B-roosters had significantly larger relative weight of crests and testes (1.63 and 1.8 times, respectively) in comparison with the progeny of M-roosters, which suggests the possibility of creating a population of Leghorn with new morpho-physiological characteristics. B- and M- roosters differ in the severity of the correlations between the weight of the body, crest and testes. In B- and M- roosters, the correlation of the relative weights of crests and C/T is positive, statistically significant. The correlation of the relative weight of the testes and C/T in both groups is negative, however, in B-roosters it is statistically significant degree correlation, and in M-roosters – weakly expressed linear correlation.