The Mapleson A, B, C and D circuits can be changed into non-polluting circuits by employing continuous gas evacuation directly from the circuit, via an ejector flowmeter (Jørgensen 1974); Mapleson A and C circuits with this modification have been described previously as the Hafnia A and C circuits (Christensen 1976, Thomsen & Jørgensen 1976). If evacuation from a closed reservoir is employed, total removal of the expired and surplus gases from the operating theatre is obtained (Jørgensen & Thomsen 1976). There will be resistance to expiration in all the circuits with a relief valve for the discharge of surplus gas. If surplus gas is continuously removed directly from the anaesthetic circuit, the patient breathes in an air compartment at ambient pressure, as long as the removal rate equals the inflow of fresh gas. The relief valve is only included in the circuit to ensure that high pressures do not develop. As in any other circuit, the relief valve remains open except during controlled ventilation. A dumping valve may also be included as a safeguard against low pressures (Jørgensen & Thomsen 1976). The flow requirements of the Hafnia B and D circuits and the corresponding Mapleson circuits have been studied in conscious, spontaneously breathing subjects, and the results are discussed in relation to the flow requirements of other semi-closed system.