Previous research has established that exposure to novel male mice can disrupt intrauterine implantation of fertilised ova in inseminated females and that much of this effect is mediated by factors in the male urine. The present studies were designed to examine whether the steroid content of male urine is sufficient to account for this effect. Pregnancy was terminated by exogenous 17β-oestradiol administered intranasally on days 2–4 after insemination in doses as low as 0.14 μg/day. Enzyme immunoassay indicated that male mouse urine reliably contains unconjugated 17β-oestradiol and testosterone. A small but significant increase in the amount of urinary oestradiol was observed in males housed nearby previously inseminated females as opposed to those housed in isolation. This influence was absent in the sire and absent in novel males when the sire was also present. The quantity of active steroids in novel male urine approaches the level sufficient to account for the disruption of implantation in nearby inseminated females.