We wished to see if chronic alterations in Na-K-ATPase activity in the medullary thick ascending limb would modify the susceptibility of its cells to the hypoxic injury produced by perfusion of the isolated kidney. Rats were fed a diet high (64%) or low (8%) in protein for three weeks. Renal medullary Na-K-ATPase was 75 +/- 12 U/mg protein/hr (mean +/- SE) in the high protein group and 44 +/- 3 in rats given low protein. After 90 minutes of perfusion, the kidneys of rats fed a high protein diet showed almost all mTAL cells near the inner medulla with severe damage (93 +/- 4.8%), whereas the same zone in perfused kidneys of rats on a low protein diet showed only 47 +/- 7.7% injury. In a similar fashion, damage to mTAL cells seen in perfused kidneys was greatly augmented by compensatory renal hypertrophy produced by removal of the contralateral kidney two weeks earlier, and by a diet high in potassium given for two weeks, procedures which also increased the activity of medullary Na-K-ATPase. The results suggest that the level of transport work of medullary cells mediated by Na-K-ATPase is a determinant of the vulnerability of mTAL cells to hypoxic injury.