The relationship between parental responsiveness of bank vole males and their body weight, testes mass, and plasma and gonadal testosterone levels were examined. Two groups of voles were studied: Group I consisted of 14 breeding pairs where females gave birth to young within a month after formation of pairs, and Group II represented 13 pairs without offspring and signs of pregnancy in females. Males in Group I, unlike those in Group II, had contact with pups before the tests on parental responsiveness. In Group I, males were found to have larger testes and to exhibit a higher level of care-giving activity (pup retrieval) as compared to males in Group II. Both the plasma and gonadal testosterone levels in males exhibiting pup retrieval were revealed to be significantly higher than those for males exhibiting infanticide. Thus, promiscuity, competition for receptive females, and a higher level of testosterone secretion, which are characteristic of bank vole males during the breeding season, are not out of the realm of possibility of the males to care of young. Our findings suggest that factors promoting parental responsiveness in bank vole males are sensitization due to contact with pups and an increase in testosterone secretion. Obviously, there is a need to reexamine the role that testoster-one plays in regulating rodent paternal behavior.