Cardiac output (CO) was measured in 10 young, healthy male subjects during rest and submaximal exercise on a bicycle ergometer by rebreathing a 2.0-2.8 l (ATPS) gas mixture of acetylene (0.7-1.2%), freon-22 (3-4.2%), argon (6-7%), and oxygen (ca. 40%) in nitrogen. End tidal gas fractions were measured by a mass spectrometer. Argon was used as an inert, insoluble gas for corrections of end tidal acetylene-, freon-, and oxygen fractions. The acetylene results corresponded to cardiac outputs found in literature (6.06 +/- 0.20 l/min, at rest and 15.05 +/- 0.44 l/min at 150 W). The freon values followed those of acetylene but were systematically lower by 0.74 l/min at rest and 1.20 l/min at 150 W. A forced respiratory rate (30-32/min) increased CO and VO2 during rebreathing at rest and lower exercise levels, while a spontaneous respiratory rate (14/min at rest and 22/min at 150 W) did not change VO2 during rebreathing compared to Douglas measurements at steady state. We conclude that freon can be used as the inert, soluble gas in the rebreathing procedure and recommend a spontaneous respiratory rate.