ABSTRACT
Pregnancy block was first observed by Bruce (1959, 1960) in labora tory mice. She noted that exposure to a strange male (or to his odor) prevented implantation in recently inseminated females and caused a return to estrus 4-5 days later. Investigations of the generality of male-induced pregnancy block (Bruce effect) indicate that it is exhibited under laboratory conditions in numerous species of rodents and may occur in natural populations as well (see following).
