Pressure and flow were recorded from the dorsal aorta of freely moving Octopus vulgaris at rest and in exercise. Mean blood pressure, the amplitude of pressure pulses and flow all double as the animal moves about. Frequency changes little in exercise, indeed the increased pulses may be associated with a small drop in heartbeat frequency. There is little or no scope for expansion of arteriovenous oxygen content difference. So the increase in cardiac output necessitated by exercise must be achieved almost solely by an increase in the stroke volume of the systemic heart. Jet propulsion is accompanied by cardiac arrest. Measurements of internal mantle pressures during jetting and of the pressure pulses generated by peristalsis of the great veins suggest that the venous system is incapable of returning blood against the gradients produced by the rise in mean mantle pressure. Since the oxygen debt that can be sustained by Octopus is quite small (?22 ml O2 kg?1), jet-propelled movement is impossible for distances of more than a few metres. Problems of the maintenance of a circulation in an animal with a high metabolic rate and the basic body plan of a mollusc are discussed, and the situation in squid is compared with that in octopuses. <br/
