The accuracy of a computerized metabolic system, using inspiratory and expiratory methods of measuring ventilation, was assessed in eight male subjects. Gas exchange was measured at rest and during five stages on a cycle ergometer. Pneumotachometers were placed on the inspired and expired side to measure inspired (I) and expired ventilation (E). The devices were connected to two systems sampling expired O₂ and CO₂ from a single mixing chamber. Simultaneously, the criterion (Douglas bag, or DB) method assessed E and fractions of O₂ and CO₂ in expired gas (FE_O2 and FE_CO2) for subsequent calculation of O₂ uptake (O₂), CO₂ production (CO₂), and respiratory exchange ratio. Both systems accurately measured metabolic variables over a wide range of intensities. Though differences were found between the DB and computerized systems for FE_O2 (both inspired and expired systems), FE_CO2 (expired system only), and O₂ (inspired system only), the differences were extremely small (FE_O2 = 0.0004, FE_CO2 = 0.0003, O₂ = 0.018 l/min). Thus a computerized system, using inspiratory or expiratory configurations, permits extremely precise measurements to be made in a less time-consuming manner than the DB technique.