The Komesaroff machine is a low-flow, closed-circle circuit with a low resistance vaporiser, of the Goldman type, in circuit. This trial assessed the mechanical consistency of the delivery of isoflurane by the vaporisers, and used six dogs to compare the in vivo cardiorespiratory effects of the anaesthetic agent delivered by the Komesaroff machine with the effects of a circle system with high flows in the semi-closed mode. The delivery of isoflurane was constant for each vaporiser setting and no potentially dangerous concentrations of isoflurane were observed. The mean (sem) percentages of isoflurane were 0.18 (0.019) at setting I, 1.46 (0.055) at setting II, 3.12 (0.066) at setting III and 3.01 (0.047) at setting IV. There were no significant differences between the two types of circuit in vivo, and the measured haemodynamic variables were satisfactory throughout the experiments.