1. A model of controlled hypoxia in the isolated perfused rat kidney has been used to compare the extent of reduction in the steady-state level of adenosine 5'-triphosphate (ATP) from that initially observed with alterations in renal function and with the development of tubular cell injury. 2. ATP depletion was observed in response to decreased total oxygen delivery even when delivery greatly exceeded consumption and the venous oxygen tension remained in excess of 150 mmHg. 3. Increases in the fractional excretion of sodium occurred progressively below an apparent threshold value of whole kidney ATP of approximately 80% of the baseline. 4. With modestly decreased oxygen delivery, cellular injury was confined to deep proximal tubule and medullary thick ascending limb of Henle's loop. Severely decreased oxygen delivery rates were associated with cellular damage spreading throughout the cortex. 5. Even the smallest reductions in whole kidney ATP were associated with morphological damage to tubular cells. The extent of reduction in whole kidney ATP was closely correlated and approximately equivalent to the calculated volume of injured cells. 6. Our results indicate that reduction in whole kidney ATP during decreased oxygen delivery is a valid marker of the extent of injurious cellular hypoxia and are consistent with the view that cellular ATP concentrations in hypoxia are markedly inhomogeneous. They support the hypothesis that specific regions of the perfused kidney become critically hypoxic and develop cellular injury while overall oxygen delivery remains high. Areas at risk include deep proximal tubule as well as the medullary thick ascending limb of Henle's loop.