Heat stressed dairy cows on pasture will compete for resources that aid cooling, but it is not known how heat stress affects the competition for water by indoor housed cows. The objective of this study was to evaluate how heat stress affects the behavior of indoor, loose-housed dairy cows at the drinker. For 3 wk after calving, cows were housed in a dynamic group (n = 20) in a pen equipped with 12 electronic feed bins, 2 electronic water bins, and 24 freestalls. A total of 68 lactating, Holstein dairy cows were enrolled over a 59-d period. The electronic water bins recorded time spent at the drinker, frequency of visits, water intake, and competitive events for 24 h/d. Competitive events were quantified using the number of replacements (when one cow displaced another cow from the drinker and took her place). Heat stress was defined as a temperature humidity index (THI) ≥ 72. THI parameters examined were daily mean, minimum, maximum, and number of h ≥68 or 72. To determine if there was a lag effect of heat stress on drinking behavior, we examined the effects of this on the day of behavioral recordings and on the previous 1 to 3 d. For the analysis of time spent at the drinker, frequency of visits, and water intake, the measures from all cows in the pen (n = 20) were averaged to create one observation per day. For the analysis of competitive events, the number of replacements at the drinker was summed from all cows in the pen (n = 20) to create one observation per day. A linear regression analysis was performed to determine the relationship between heat stress and behavior at the drinker. We found that, with increasing numbers of hours in heat stress conditions (THI ≥72) over a 3-d period, cows drank more water, spent more time at the drinker, made more visits to the drinker, and engaged in more competition (P <0.0001 for all). These results indicate that behavior at the drinker may provide a biomarker of heat stress. This measure may be of practical value, especially for farms where attendance at the drinker can be monitored electronically.