The effects of phosphate depletion (PD) of 4, 8, and 12 weeks on skeletal muscle energy metabolism were studied in rats fed a phosphate deficiency diet and compared with rats pairfed with a normal phosphate diet. Skeletal muscle biopsy specimens were examined for energy production, transport, and utilization. The results show that already by 4 weeks of PD, the concentration of inorganic phosphorus of the skeletal muscle was significantly reduced and remained low thereafter. There was significant (P less than 0.01) and direct correlation between the cellular inorganic phosphorus and that of serum phosphorus. Adenine nucleotides, ATP, ADP, AMP, and creatine phosphate levels did not change. Mitochondrial respiration and oxidative phosphorylation were impaired by PD. Total cellular mitochondrial and myofibrillar creatine phosphokinase activities were significantly reduced at 4 weeks of PD and fell further at 8 and 12 weeks. There was a significant (P less than 0.01) and direct correlation between the activity of total extractable creatine phosphokinase and both serum and cellular levels of inorganic phosphorus. These data show that chronic PD is associated with a decrease in energy production, transfer, and utilization by skeletal muscle and provides information on the molecular events responsible for the myopathy of PD.