The humidity output and CO2 elimination of the Bain circuit were tested on a simulated adult patient. The moisture content of inspired gases was found to be adequate when the circuit was used with a minute volume of 8.4 L/min and a fresh gas inflow of 4.9 L/min (65 percent relative humidity at room temperature at the onset of experimentation, rising to 100 percent after 80 minutes). However, the mean inspired CO2 concentration increased from 0.8 percent to 5.5 percent when the fresh gas inflow was decreased from 8 L/min to 3.5 L/min. It is recommended, therefore, that the circuit should not be used for long periods of time without measuring arterial CO2 tension or in situations where intentional hypocarbia is desired.