Pregnancy failed in a high proportion of newly mated Microtus agrestis females when they were exposed to a strange male between 48 and 72 hr after mating with a stud male. This effect of the strange male was testosterone dependent. Direct contact with the strange male was normally necessary, and even a single barrier of wire mesh between the female and strange male prevented the male from exerting his pregnancy-blocking effect. The results suggest that the stimuli mediating pregnancy blocking in the vole may differ from those operative in the mouse.