The aim of this investigation was to study sweat production during exercise at 2 ambient temperatures (20°C and 35°C) and the concurrent localisation of carbonic anhydrase (CA) in the sweat gland. Horses develop alkalosis during prolonged exercise and the sweat contains HCO3-. Carbonic anhydrase is therefore of interest since it catalyses the reaction CO2 + H2O ↔ HCO3- + H+. Four standardbred trotters performed an exercise test. Skin biopsies were taken from the neck, and sweat rate, blood and skin temperatures were measured. There was a close relationship between sweat rate, temperatures and work intensity at 20°C. Temperatures and sweat rate were higher at 35°C and did not fall when the work intensity dropped. A significant decrease in the sweat gland cell area was found after exercise at 35°C with an accompanying decrease of vesicles. Strong CA activity was present at the luminal cell membrane and weaker basolaterally. The staining intensity increased after exercise. We suggest that CA might be of importance for counteracting the alkalosis developed after exercise by delivering HCO3- for generation of the alkaline pH in sweat.