Continuous measurement of the CO2 concentration in inhaled gas is the best method to know accurately the importance of rebreathing. Four anesthetic breathing systems were studied in 60 adult patients (Dighby-Leigh valve, circle system with and without soda lime. Waters system without soda lime, Bain circuit). Fresh gas flows (FGF) inspired and end-expired concentrations of CO2 (FI CO2, FE' CO2) and PaCO2 are measured. The FI CO2 seems to be negligible with Dighby-Leign valve and circle system with soda lime but important with other systems, even with FGF as high as 200 ml/kg, the FI CO2 is about 1 p. cent for Bain circuit, 2 p. cent in spontaneous ventilation, 2,7 p. cent in controlled ventilation for Waters valve. It is possible to theoretically determine that the FI CO2 would not exceed 1 p. cent.