A study was conducted to determine whether following exposure of male mice to high temperatures, the ability of their spermatozoa to fertilise ova was reduced, especially during the period before the males became completely infertile. Male mice placed in a microclimate chamber at 36°C for two periods, each of 12 h on successive days, were less able to fertilise control females in vivo when mated and, even in those females that became pregnant, litter size was reduced. However, these effects were associated with falls in testis weight and numbers of spermatozoa in the testis and epididymis. To determine whether the effect on fertility was a result of the decreased spermatozoa numbers, spermatozoa were collected from the epididymides of heated and control males. Equal numbers of motile spermatozoa from an unselected sample or those subjected to a swim-up procedure to separate those that were motile from the immotile ones in the sample were then mixed in vitro with oocytes from superovulated normal females. Similar numbers of spermatozoa from both control and heated males bound to the zona pellucida but smaller percentages of the oocytes were fertilised by spermatozoa from the heated males and fewer of these spermatozoa penetrated the ova. The effects were first seen 7 days after the heat exposure and became more obvious after 10 or 14 days.