The influence of biologically active substances of excretory products (EP) of house mouse adult males on reproductive function of young males was studied with the help of the dominant lethal test. Mortality of the progeny and frequency of nonpregnant females were analysed on the 6th and 8th weeks after EP exposure. Preimplantation and postimplantation mortality frequency increased significantly (P less than 0.05) at 6th and 8th weeks after 2 h EP exposure. Frequency of dominant lethals and nonpregnant females is also increased. Thus, inhibition of reproductive function of young males after EP action has been demonstrated. Possible mechanisms of the effects revealed and their importance for house mouse populations are discussed.