Male mouse urine contains 17β-oestradiol (E2) and other steroids. Given that males actively direct urine at proximate females and intrauterine implantation of blastocysts is vulnerable to minute amounts of exogenous oestrogens, males' capacity to disrupt early pregnancy could be mediated by steroids in their urine. When male mice were implanted with osmotic pumps containing tritium-labelled E2 (3H-E2) or injected i.p. with 3H-E2, radioactivity was reliably detected in their urine. Following intranasal administration of 3H-E2 to inseminated females, radioactivity was detected in diverse tissue samples, with there being significantly more in reproductive tissues than in brain tissues. When urine was taken from males injected with 3H-E2, and then intranasally administered to inseminated females, radioactivity was detected in the uterus, olfactory bulbs, and mesencephalon and diencephalon (MC+DC). When inseminated and ovariectomised females were perfused at the point of killing to remove blood from tissues, more radioactivity was detected in the uterus than in muscle, olfactory bulbs, MC+DC and cerebral cortex. Pre-treatment with unlabelled E2 significantly reduced the uptake of 3H-E2 in the uterus. Taken with evidence that males deliver their urine to the nasal area of females, these results indicate that male urinary E2 arrives in tissues, including the uterus, where it could lead to the disruption of blastocyst implantation.