Ernst All-Russian Research Institute of Animal Husbandry, Federal Agency of Scientific Organizations, pos. <...> Dubrovitsy, Podolsk Region, Moscow Province, 142132 Russia; 2Head Center for Reproduction of Farm Animals OAO, 3, ul. <...> Bykovo, Podolsk Region, Moscow Province, 142143 Russia, e-mail komnina@list.ru Received May 13, 2016 A b s t r a c t Wide use of artificial insemination necessitates a deeper understanding of how hormones and immune system influence on semen production in sires. <...> With this regard, the role of follicle stimulating hormone and luteinizing hormone in spermatogenesis is under consideration. <...> Blood testosterone and estradiol levels are related and depend on a testosterone-estradiol binding globulin function. <...> We first examined seasonal effects of blood estradiol levels as a reproduction marker in Holstein bull sires and showed the relationship between the blood estradiol concertation, semen fertilizing ability, semen volume, and semen concentration which, in turn, impact on the results of artificial insemination. <...> Estradiol level in Holstein sires aged 30±6 months (n = 18) was assayed using Immuno-FA-E ELISA kit and a Uniplan equipment (ZAO Pikon, Russia). <...> We showed that in the bulls the blood estradiol level varied significantly depending on a season (Р < 0.001). <...> In spring, the lowest (0.100 nmol/l) estradiol level detected in 78 % of the bulls was mostly characteristic, and only in 17 % of the bulls estradiol was beyond 0.200 nmol/l (Р < 0.001). <...> At autumn, blood estradiol concentration increased in 94 % bulls (Р < 0.001). <...> When low blood estradiol on the daу of semen collecting, a 12-17 % success rate occurred in cows after a single insemination (Р < 0.05), and 17-29 % heifers became pregnant to first insemination (Р < 0.001). <...> Keywords: estradiol, bull sires, seasonal changes, semen indices, success rate of insemination For effective breeding and artificial insemination of farm animals, the rational use of sires' sperm [1] should be employed, which requires, in turn, an indepth research of the role of the hormonal and immune systems in the sperm production. <...> In all mammalians, spermatogenesis is modulated by peptide and steroid hormones, such as folliclestimulating hormone (FSH) and luteinizing hormone <...>