Despite speculation about the swimming efficiency of cetaceans, few studies have investigated the exercise physiology of these mammals. In view of this, we examined the physiological responses and locomotor energetics of two exercising adult Tursiops truncatus. Oxygen consumption, heart rate, respiratory rate and post-exercise blood lactate concentration were determined for animals either pushing against a load cell or swimming next to a boat. Many of the energetic and cardiorespiratory responses of exercising dolphins were similar to those of terrestrial mammals. Average heart rate, respiratory rate and oxygen consumption for dolphins pushing against a load cell increased linearly with exercise levels up to 58 kg for a female dolphin and 85 kg for a male. Oxygen consumption did not increase with higher loads. Maximum rate of oxygen consumption (VO2max) ranged from 19.8 to 29.4 ml O2 kg-1 min-1, which was 7-11 times the calculated standard metabolic rate (VO2std) of the dolphins. Blood lactate concentration increased with exercise loads that exceeded VO2max. The maximum lactate concentration was 101.4 mg dl-1 (11.3 mmol l-1) for the male, and 120.6 mg dl-1 (13.6 mmol l-1) for the female. When swimming at 2.1 m s-1, heart rate, respiratory rate and post-exercise blood lactate concentration of the dolphins were not significantly different from values at rest. The cost of transport at this speed was 1.29 +/- 0.05 J kg-1 min-1. The energetic profile of the exercising bottlenose dolphin resembles that of a relatively sedentary mammal if the exercise variables defined for terrestrial mammals are used. However, the energetic cost of swimming for this cetacean is low in comparison to that of other aquatic and semi-aquatic mammals.