Radiographs of live, unanesthetized snakes were used to document the position of the heart in the body cavity during horizontal, head-up, and head-down postures. The extent of cardiac displacement observed during these postural changes differed substantially among the snakes examined, ranging from virtually none in a thin-bodied arboreal snake to as much as three vertebral lengths (= half the length of the heart) in a heavy-bodied terrestrial Crotalus. The basis of this differential cardiac displacement is attributed to the anatomical "packaging" of the pericardial sac. In some snakes the pericardial sac is loosely suspended in the body cavity by the great vessels and connective tissue sheets. In contrast, in other snakes the pericardical sac is buttressed against the body wall, the lung, or the liver. We hypothesize that cardiac displacement during postural change may alter the pattern of blood flow in the aortae of snakes.