The activities of two ATP-requiring systems with different subcellular localizations were studied in cells in which average cellular ATP concentration was varied. The cytosolic ATP-sulfurylase activity varied linearly with the cellular ATP concentration; however, the plasma membrane Na+-K+-ATPase was substantially more sensitive to decreased ATP concentration. Under conditions where the cellular ATP concentration was lowered to 40% of control, Rb+ uptake was nearly zero. The results indicate that ATP-utilizing enzymes located in the plasma membrane in liver cells are exposed to a lower ATP concentration than are enzymes present in the cytosolic fluid surrounding the mitochondria. Analysis of radial diffusion of ATP from mitochondria, assuming that mitochondria are spherical and ATP consumption is zero order in ATP concentration, shows that the average ATP supply radius decreases as mitochondrial ATP production decreases. Hence, during limited ATP supply, enzymes with a greater average distance from mitochondria (e.g., plasma membrane ATPases) experience a more dramatic decrease in ATP concentration than enzymes in close proximity to mitochondria (e.g., cytoplasmic enzymes). Thus microheterogeneity of ATP supply can occur in cells without membranal compartmentation due to nonuniform distribution of ATP-generating and ATP-consuming systems.