A number of respiratory and circulatory parameters was measured in the Larger Spotted Dogfish Scyliorhinus stellaris before, during and after periods of spontaneous swimming. During swimming the gill ventilation was increased, mainly due to increased ventilatory stroke volume, the respiratory frequency showing a small rise only, and the cardiac output was increased at only slightly elevated cardiac frequency. Coordination between cardiac, ventilatory or locomotor (tail-beat) rhythms was not observed. The decrease in utilization of inspired water O2 during swimming was attributable to diffusion limitation in branchial O2 transfer. A considerable fraction of the total net amount of O2 required for swimming was taken up during the recovery phase. From the observations that (1) the decrease in gill ventilatory flow after cessation of swimming revealed a very rapid component (followed by a slow component), and that (2) changes in swimming speed were reflected by immediate changes in momentary ventilatory flow, it is concluded that the increased ventilation during swimming was in part mechanical-passive and/or due to nervous coupling between respiratory and locomotor centers.