Most of silver foxes bred in captivity show predominantly defensive responses to human contact. Since early captive breeding humans have unconsciously carried out selection for tameness. Thus, artificial selection for the absence of aggressive and fearful responses to humans most likely played a key role in domestication process. To establish the genetic and physiological mechanisms of the early evolution of domestic animals, a model of silver fox domestication was developed at the Institute of Cytology and Genetics (Novosibirsk, Russia). A population of silver foxes has been produced in long-term selection for lack of aggression and fear towards humans (domestic behaviour). The method of selection and the behavioural changes in the course of selection have been described (Belyaev, 1979; Trut, 1995). Selected animals show no aggressiveness to humans and they are better adapted to captive conditions than their wild counterparts. The purpose of this study was to determine how behaviour selection influences testicular function in silver fox males. Plasma concentrations and testicular production of testosterone were determined in selected and control males in different parts of the reproductive cycle and prenatal life. In addition, reproductive behaviour and hormonal responses to opposite sex were estimated in the males of both behaviour groups. Sperm production and sexual activity were also compared between selected and control males.