Changes in the cardiorespiratory function and the metabolic responses induced by walking exercise (40 m/min, 80 m/min) in Holstein steers in thermoneutral (18°C) and hot (33°C) environments were investigated. Cardiac output and stroke volume at rest, during and after exercise in a hot environment, except during exercise at 80 m/min, were larger than in a thermoneutral environment. Stroke volume was larger in a hot environment and during exercise, whereas the mean arterial blood pressure was not different between thermal environments, or exercise levels. Total peripheral resistance was lower in a hot environment than in a thermoneutral one, and it also decreased in both environments when the exercise was imposed on the steers. Respiratory rate and respiratory minute volume during the resting period were larger in a hot environment than in a thermoneutral one. However, the extent of the increase in both parameters induced by the exercise was not affected by the environmental temperature. After exercise in a hot environment, alveolar ventilation reverted to the initial levels within 5 min, while both respiratory minute volume and dead space ventilation still remained at higher levels. These results indicate that the change in the respiratory minute volume after exercise in a hot environment was due mainly to the change of the respiratory dead space. The higher the walking speed was, the more the heat production increased during exercise. Also, heat production during exercise was somewhat higher in a hot environment than in a thermoneutral one.