Cows must be observed in estrus so they can be inseminated artificially. There is clear evidence that heat stress decreases the length and intensity of estrus. For example, Nebel et al. (1997) demonstrated that dairy cows in the summer had approximately one-half the number of mounts per estrus compared to dairy cows in the winter. Thatcher and Collier (1986) demonstrated a similar change in Florida dairy cows. The intensity of estrous expression is increased when heat- stressed cows are cooled. Heat stress decreased follicular estradiol in some studies (Wilson et al., 1998ab). Therefore, the decrease in estrous intensity may be caused by a decrease in follicular estradiol secretion. Although this is an attractive hypothesis, the blood estradiol concentrations needed to trigger behavioral estrus are poorly defined. Thus, it is impossible to say whether or not heat-stressed dairy cows fail to reach a minimum threshold of estradiol for expression of estrus. An equally likely cause of reduced estrous expression is the physical inactivity caused by heat stress. Cows are less active during heat stress and therefore less likely to ride other cows during estrus