1. The anatomy of the counter-current heat exchanger of skipjack tuna is described and the pattern of blood flow is analysed. 2. The pattern of blood flow is from the dorsal aorta, through the exchanger to segmental arteries to the tissues, from the tissues to segmental veins and back through the exchanger to the post-cardinal vein. 3. The vessels in the exchanger are about the same size as systemic arterioles and venules and are about 10 mm long. There are about 125000 of each type in a 2 kg tuna. 4. The velocity of blood flow in the exchanger is about 1/80th of that in the dorsal aorta and post-cardinal vein allowing time for heat transfer. 5. There are many valves in the segmental veins which may be expected because of the resistance offered by the exchanger. 6. The vessels in the tuna heat exchanger are an order of magnitude larger than those in the swim-bladder rete, thus permitting heat transfer but preventing gas transfer.