The detailed distribution of water, Na, and K in a large series of old rats (40) twenty to twenty-five months of age was compared with that of young rats five to seven months old. Measurements were made in plasma and in gastrocnemius muscle samples. There was a marked increase in the extracellular space in old rats mostly due to a shift of water from cells to environment. This is accompanied by a marked gain in total Na and a corresponding loss of total K. The gain of Na is predominantly extracellular but also affects cells. The loss of K is entirely cellular. Increases of Na and K in the extracellular space parallel the increase in water so that no real change in their plasma concentration occurs. The fall in cell K likewise parallels the decrease in cell water. By contrast, the gain in cell Na coupled with the loss of cell water produces a sizable increase in cell Na concentration and a marked fall in the transcellular Na concentration gradient, [Na]o/[Na]i.