Sepsis increases phosphocreatine (PCr) breakdown and reduces PCr stores in skeletal muscle. To determine if systemic infection impairs mitochondrial function, in vivo 13P magnetic resonance spectroscopy (31P MRS) studies of the gastrocnemius muscle were performed in virus-free male Wistar rats 24 or 48 hr after cecal ligation and 18-gauge needle single puncture (24 degrees CLP, n = 16; 48 degrees CLP, n = 15) or sham operation (24 degrees SHAM, n = 18; 48 degrees SHAM, n = 13). Physiologic saline (6 ml/100 g body wt) was injected intraperitoneally for fluid resuscitation. Water but no food was allowed in all animals. High resolution (8.45 Tesla) 31P MRS spectra, obtained at rest and during exercise using a 1.4-cm surface coil, were used to calculate PCr/ATP, PCr/P(i) ratios, and intracellular pH. Steady-state muscle exercise was induced by supramaximal sciatic nerve stimulation at 10 Hz for 10 min. Recovery of PCr/(PCr + P(i)) ratios after exercise was fitted to a monoexponential curve. The resultant function was used to calculate the half time for PCr recovery, the initial PCr resynthesis rate, and the maximal oxidative ATP synthesis rate, which reflect the rephosphorylation of ADP and are therefore measures of mitochondrial oxidative capacity. PCr/ATP ratios decreased by 12 and 11%, 24 and 48 hr after CLP, respectively. The PCr/P(i) ratios decreased incrementally (7% in 24 degrees CLP vs 23% in 48 degrees CLP animals). Twenty-four hours after operation the half time for PCr recovery was shortened while the initial PCr resynthesis rate and maximal oxidative ATP synthesis rate were accelerated in CLP animals compared to controls.(ABSTRACT TRUNCATED AT 250 WORDS)