1. Researches were made on 50 hearts of 9 different Pinnipeds, of the Hippopotamus and the Otter. 2. In the Northern Fur Seal, the Grey Seal, the Ringed Seal, the Bearded Seal, the Banded Seal and the Walrus the ductus arteriosus closes anatomically at the same time, or even earlier, than in terrestrial Mammals of comparable size. In contrast to those of the Common Seal the newborn pups of these animals remain on land for several weeks, or, if they swim during the first weeks after birth, they do so infrequently and they do not dive. These data confirm the supposition that the retarded anatomical closure of the ductus arteriosus in Cetaceans and in the Common Seal may be explained by the fact that their newborns swim and dive immediately or almost immediately after birth. Shortage of oxygen (or eventually a rise of the pressure in the pulmonary artery) may cause a temporary re-opening of the ductus during the first period of postnatal life, which retards its anatomical closure. 3. All Pinnipeds show a more or less retarded closure of the foramen ovale during the first period of postnatal life. Obviously this phenomenon is not directly related with the closure of the ductus arteriosus. 4. In most Pinnipeds a ramification of the aortic arch was found, which corresponds with the situation in Man. Some animals show a more concentrated type, like the Dog and the Pig. The Walrus has a more dispersed type. 5. Data are given about the shape of the heart. This shape and the type of ramification of the aortic arch are highly correlated with the spatial relations in the thoracic cavity. 6. The ratio of the thickness of the right and left ventricular wall in Pinnipeds is the same as in terrestrial Mammals. This is the case in newborns as well as in young and adult animals.