We describe a simple and reproducible procedure for measuring maximal O2 consumption (Vo2max) that can be used for mammals spanning a wide range of body mass (Mb). Vo2 of trained animals was measured as a function of speed while they ran on a treadmill. Lactate concentration of the blood was determined at the beginning and end of the run. We considered that an animal had achieved Vo2max when Vo2 no longer increased with increasing tread-speed and the additional energy consumed by the muscles could be accounted for by anaerobic glycolysis. We found that Vo2 increased linearly with tread-speed up to a maximal rate and then remained unchanged with further increases in speed in nine species. When the animals ran at speeds faster than that where Vo2max was reached, they refused to continue when blood lactate concentrations reached 18-28 mmol . kg-1. Values for Vo2max obtained with this procedure were found to be about 20% greater than values obtained with the 'cold exposure' procedure for determining Vo2max. We conclude that our treadmill procedure is valid for determining the allometric relationship between Vo2max and Mb.