The purpose of these experiments was to compare two systems for measuring oxygen uptake (VO2): the open circuit method of indirect calorimetry (Rv), which is commonly used in human studies, and the open flow method, which has been adopted in a variety of animal studies. VO2 measured using the Rv system (procedure 1) was accepted as the criterion value. Two procedures were used to calculate VO2 in the open flow system: MCO2 (where O2 and CO2 were measured in mixed expired gas) and MO2 (O2 measured only). VO2 was measured in ten human subjects during three continuous incremental cycling tests to peak power output administered on three separate occasions--once using each of the three procedures. The results indicated a relatively small difference in VO2 between the Rv and MCO2 procedures (6.2 +/- 4.5%) and between the Rv and MO2 procedures (3.7 +/- 1.7%), across a broad range of power outputs (40-100% VO2 peak). The difference between the Rv and MCO2 procedures decreased as power output increased, while that between the Rv and MO2 procedures remained relatively constant. Therefore, the open flow method is shown to be a valid and reproducible technique for assessment of the metabolic activity of humans and animals.