Isolated rabbit retinas were subjected for various durations to several types of ischemic insult, and then returned to control medium for periods of up to 4 3/4 h before measurements were made of total water, inulin-free water, and plasma membrane permeability as assessed by mannitol penetration into the inulin-free water. Neither anoxia nor substrate deprivation alone, for as long as 50 min, caused significant irreversible swelling, but they were synergistic in combination. Restricting the volume of extracellular fluid during the combined deprivation caused the changes responsible for swelling to occur much sooner. There was a progressive increase in membrane permeability, with a delayed increase in intracellular water beginning about 2 h after the ischemic insult. Cell swelling correlated closely with loss of viability as evidenced by failure to reinstitute protein synthesis, but the swelling appeared to be the consequence rather than the cause of the initial irreversible damage.