The minimum resistance of expiratory Heidbrink-type valves has been assessed by determining the pressure decrease across the valves at a flow rate of 30 litre min−1. The average resistance of 70 valves currently in clinical use was 318 Pa at that flow rate and about 44% had resistances greater than the limit of 294 Pa previously suggested. The resistances obtained in new valves were either similar or fractionally greater. Valves being introduced as parts of scavenging systems appeared, on the whole, to have lower resistances.