1) control cows and 2) mineral-treated cows. All cows were exposed to environmental heat stress conditions (i.e., temperature-humidity index = 79.4 ± 4.3 units). No study variable was affected (P ≥ 0.20) by the treatment x sampling day interaction. While the mineral supplement did not affect any physiological variable in the afternoon, this treatment decreased breaths per min (P = 0.01) and most body surface temperatures (P ≤ 0.06; head, shoulder, leg, right-flank, and udder) in the morning. There was no effect (P = 0.37) of the mineral supplementation on milk yield but increased (P ≤ 0.03) the percentages of solids non-fat, protein, lactose, and density in the milk. In conclusion, Holstein cows' physiological thermoregulation and milk composition experiencing summer heat stress were improved by applying an injectablemineral supplement.(AU)