A 4-wk training program was undertaken by 15 untrained non-heat-acclimated males who were divided into three groups matched on maximal aerobic capacity (V̇(O2)(max) and trained either in water or on land to determine how physical training (PT) in these different media affects heat tolerance. Subjects trained on a cycle ergometer for 1 h/day, 5 days/wk at 75% V̇(O2)(max), with the exercise intensity progressively increased to maintain a constant training stimulus. Group I exercised on land, whereas groups II and III exercised while immersed to the neck in water of either 32°C (II) or 20°C (III). Daily exercise increased core temperature (T(c)) in groups I and II but not in group III. Training elicited similar increases (~15%) in V̇(O2)(max) in the three groups. Before and after PT, all subjects exercised at ~30% V̇(O2)(max) for 3 h at 49°C, 20% rh. Compared with before training, groups I and II showed a decrease in final T(c) and heart rate (HR) in the posttraining heat exposure. Sweat rate increased 25% in group II but remained the same in group I. Group III demonstrated a decrease in final HR, but final T(c) was higher than before training. Sweat rate did not increase in group III and was lower than the other groups. It was concluded that PT can improve the cardiovascular response to dry heat without affecting thermoregulatory capacity. PT appears to enhance heat tolerance only if T(c) is permitted to rise during exercise, thus stimulating the temperature-regulating center for heat dissipation.