1. Cardiovascular dynamics and the functional status of a double circulation have been studied in representatives of the three genera of lungfishes; Neoceratodus, Lepidosiren, and Protopterus. 2. The experimental approach consisted in continuous recording of heart rate, blood pressures and blood velocity from appropriate blood vessels in intact, unanesthetized fish, free to swim in large aquaria. Blood gas analyses were done in all species on repetitive samples from central blood vessels including pulmonary arteries and veins, coeliac artery, vena cava and afferent branchial arteries. 3. Branchial vascular resistance in Neoceratodus compares with teleost and elasmobranch fishes and correlates with a dominance of aquatic gill breathing in the bimodal gas exchange (Kg. 2). In Protopterus aerial breathing dominates and branchial vascular resistance is low in accordance with a general reduction in aquatic gas exchange and branchial vascularization. The small branchial vascular resistance varied with external conditions in apparent relation to the usefulness of the remaining branchial exchange circulation (Figs. 7 A and B). 4. Branchial vascular resistance increased in response to intravenous injections of acetylcholine, while adrenalin had a vasodilatory effect on branchial vessels in Neoceratodus and Protopterus; the two species studied to this effect (Figs. 19A and 20). 5. Venous return in all species depended on suctional attraction by the heart in addition to the driving force from the arterial side. Suctional attraction tended to be more important in the systemic than in the pulmonary veins (Figs. 12A and B). 6. In all species arterial systolic pressures and pulse pressures were higher in systemic than pulmonary arteries. Arterio-venous pressure difference and vascular resistance were consistently lower in the pulmonary than the systemic circuit (Figs. 5A and B, 8A and B). While resting in aerated water Neoceratodus had higher arterial pressures than Protopterus and Lepidosiren. 7. Blood velocity measurements were done in Protopterus. Blood velocity in the distal bulbus cordis segment was commonly discontinuous, but the ejection phase was prolonged by elastic recoil and contraction of the bulbus cordis, resulting in positive outflow throughout most of the cardiac cycle (Fig. 10). Pulmonary arterial blood velocity was continuous, commonly with a high diastolic velocity component (Fig. 11). Blood velocity in the vena cava and pulmonary vein was variable (Figs. 12A and B). 8. Spontaneous and artificial lung inflation elicited increased cardiac output and an increased heart rate and arterial blood pressure. The response appeared to be of reflex character (Figs. 16, 17). Voluntary airbreaths were regularly associated with marked shifts in regional blood flow increasing the proportion of pulmonary flow to total cardiac outflow. Swimming movements similarly elicited marked adjustments in regional blood flow (Fig. 18). 9. Blood gas analysis were done on all species and documented a clear tendency for preferential circulation of oxygenated and deoxygenated blood in Protopterus and Lepidosiren (Table). The extent of preferential circulation depended upon the intensity of airbreathing and the phase of the interval between airbreaths (Fig. 22)