During those dog days of late summer and the end of the breeding season, most of us who work in equine reproduction are ready for a break. You are left with those difficult mares that didn’t settle earlier in the season and stallions that may be growing tired of breeding. Mares may not cycle as predictably or conceive as readily and stallions may be tougher to collect or exhibit a decrease in semen quality. One of the factors that can contribute to these problems is heat stress. As ambient temperatures rise, stallions and mares may experience disruptions to normal reproductive function as a result. This article will briefly describe some of the effects of heat stress on mare and stallion reproductive function.