Uptake of 1 per cent ethylene was studied in eight healthy, awake, unpremedicated adults. End-expiratory concentrations of ethylene were measured by gas chromatography, and CO2by infrared analysis. In all subjects, end-tidal concentration of ethylene was at least 95 per cent of equilibrium at the end of five minutes, 97 per cent by 10 minutes and 99 per cent at the end of 20 minutes. Studies were terminated at 30 minutes because of inability to detect differences between inspired and expired ethylene concentrations. It was unlikely that total body saturation had occurred in that time. Ordinate-derivative analysis of the individual experimental curves yielded two exponentials of the general formae−kt. Neither exponential could be related to a specific body compartment. Comparison of two separate studies done on the same subject, measuring “wet” end-tidal samples in one and “dry” samples in the other, showed no essential differences between the two uptake curves, indicating that the diluting effect of pulmonary water vapor on the dry inspired gas mixture could be ignored.