Metabolic rate and the relative contributions of aerobiosis and anaerobiosis to energy supply during unrestrained submergence were calculated in juvenile Crocodylus porosus. Voluntary undisturbed dives were short in duration (mean = 4.7 min ± 0.3 SEM; n = 60) and did not result in a decline in metabolic rate below predive levels. These dives were aerobic because negligible quantities of blood lactate were generated. Crocodiles surfaced before pulmonary O₂ tensions (PLo2) dropped below 70 mmHg (mean = 75 mmHg ± 4.4 SEM; n = 23), and it is proposed that metabolic rate does not decline during voluntary dives because tissues do not become hypoxic. When submergence was prolonged beyond voluntary limits, aerobic metabolism (estimated from the rate of decline in PLo2) did not decline until PLo2 fell below 50 mmHg. This suggests that internal O₂ tensions rather than submergence per se influenced aerobic metabolism during these dives. Crocodiles appear to change aerobic metabolism during submergence to meet specif...