A large, portable water tunnel has permitted insight into the swimming physiology of sharks and tuna. Tunnel use has demonstrated the capacity of sharks to sustain swimming speeds of over one body length per second. This level of aerobic performance for sharks and the measured rates of oxygen consumption during swimming both exceed previous estimates for this group, and are similar to values reported for bony fishes. By permitting studies with stably swimming, instrumented sharks, the tunnel has enabled experimental analyses demonstrating the vis a tergo cardiac filling mechanism and the role of the pericardioperitoneal canal in facilitating rapid changes in cardiac stroke volume. For tunas, the tunnel has permitted long-term and replicate studies of metabolic rate and factors affecting it (temperature, body size, velocity), of heat transfer, and of swimming kinematics. With the incorporation of anaesthesia and surgical techniques it is now possible to investigate tuna cardiorespiratory responses to changes in temperature, oxygen, and swimming velocity, and to carry out biomechanical studies that include the timing of electromyograms at sites along the body, and measurement of muscle force generation in the lateral tail tendons during swimming.