Systemic infection with Streptococcus pneumoniae produced atrophy, decreased twitch and tetanic tension, and altered intracellular electrolyte composition in rat skeletal muscle. Cathepsin B activity was selectively elevated early in the course of illness. Luepeptin, a cathepsin B inhibitor, and indomethacin, a prostaglandin synthesis inhibitor, prevented muscle atrophy and impaired contractility. Indomethacin, but not leupeptin, prevented the intracellular electrolyte changes. Acetaminophen reduced fever but did not prevent muscle atrophy, impaired contractility, or altered intracellular electrolytes. Muscle wasting and impaired contractility associated with sepsis may involve selective prostaglandin stimulation of cathepsin B activity. Intracellular electrolyte changes may involve prostaglandin synthesis but do not require cathepsin B activation.