Retinal ischemia caused by systemic hypotension produces significant functional and morphological symptoms. Animal experiments were carried out in dogs to study the retinal alterations during cardiac arrest created by artificial pericardial tamponade which lasted for 1 to 9 minutes. The main parameters of the blood circulation were recorded with a multichannel recording system and serial fundus photographs were made. The retinas, after a prompt enucleation of the eye, were studied by light and electron microscope. The retina and arterial walls became cloudy during the circulatory arrest, which were reversible, if the blood circulation was recreated not later than 4 minutes. Ultrastructural alterations can be seen in the retina after 4 minutes. Mitochondrial destruction of the ganglion cells and bipolar cells as well as edema of Muller's cells are the first consequence of the circulatory arrest, which is followed by disintegration of the capillary endothelium and destruction of the inner segment of the photoreceptor cells. These observations suggested that during circulatory arrest serious ultrastructural destructions occur both in the retina and retinal capillaries which explain the definitive diminution of retinal functions.