A B S T R A C T The basis for skeletal muscle dysfunction in phosphate-deficient patients and animals is not known, but it is hypothesized that intracellular phosphate deficiency leads to a defect in ATP synthesis. To test this hypothesis, changes in muscle function and nucleotide metabolism were studied in an animal model of hypophosphatemia. Mice were made hypophosphatemic through restriction of dietary phosphate intake. Gastrocnemius function was assessed in situ by recording isometric tension developed after stimulation of the nerve innervating this muscle. Changes in purine nucleotide, nucleoside, and base content of the muscle were quantitated at several time points during stimulation and recovery.Serum concentration and skeletal muscle content of phosphorous are reduced by 55 and 45%, respectively, in the dietary restricted animals. The gastrocnemius muscle of the phosphate-deficient mice fatigues more rapidly compared with control mice. ATP and creatine phosphate content fall to a comparable extent during fatigue in the muscle from both groups of animals; AMP, inosine, and hypoxanthine (indices of ATP catabolism) appear in higher concentration in the muscle of phosphate-deficient animals. Since total ATP use in contracting muscle is closely linked to total developed tension, we conclude that the comparable drop in ATP content in association with a more rapid loss of tension is best explained by a slower rate of ATP synthesis in the muscle of phosphate-deficient animals. During the period of recovery after muscle stimulation, ATP use for contraction is minimal, since the muscle is at rest. In the recovery period, ATP content returns to resting levels more slowly in the phosphate-deficient than in the control animals. In association with the slower rate of ATP repletion, the precursors inosine monophosphate and AMP remain elevated for a longer period of time in the muscle of phosphate-deficient animals. The slower rate of ATP repletion correlates with delayed return of normal muscle contractility in the phosphate-deficient mice. These studies suggest that the slower rate of repletion of the ATP pool may be the consequence of a slower rate of ATP synthesis and this is in part responsible for the delayed recovery of normal muscle contractility.