Male mice actively direct their urine at nearby females, and this urine reliably contains unconjugated oestradiol (E2) and other steroids. Giving inseminated females minute doses of exogenous E2, either systemically or intranasally, can cause failure of blastocyst implantation. Giving juvenile females minute doses of exogenous E2promotes measures of reproductive maturity such as uterine mass. Here we show that tritium-labelled E2(3H-E2) can be traced from injection into novel male mice to tissues of cohabiting inseminated and juvenile females. We show the presence of3H-E2in male excretions, transmission to the circulation of females and arrival in the female reproductive tract. In males,3H-E2given systemically was readily found in reproductive tissues and was especially abundant in bladder urine. In females,3H-E2was found to enter the system via both nasal and percutaneous routes, and was measurable in the uterus and other tissues. As supraoptimal E2levels can both interfere with blastocyst implantation in inseminated females and promote uterine growth in juvenile females, we suggest that absorption of male-excreted E2can account for major aspects of the Bruce and Vandenbergh effects.