Occasional case reports have shown that acute myopathy may occur in patients treated with massive doses of corticosteroids. The mechanism of this myopathy is poorly understood. Therefore, 60 male rats were randomly assigned to receive daily injection of saline (C), methylprednisolone (M), or triamcinolone (T) 80 mg/kg/d for 5 d. Nutritional intake, measured daily in 15 animals, showed a significant reduction of food intake in the steroid-treated groups (-50 and -79% in M and T, respectively). This was associated with a similar loss in body weight. In the 45 remaining animals, diaphragm contractility and histopathologic features of several muscles were studied. Weights of respiratory and peripheral muscles were similarly decreased after steroid treatment. Maximal twitches of the diaphragm were lower in the C group (653 +/- 174 g/cm(2)) than in the M group (837 +/- 171 g/cm(2); p < 0.05) and the T group (765 +/- 145 g/cm(2), NS). Half-relaxation time was prolonged in both steroid groups, and time to peak tension was longer with M, whereas tetanic tensions were similar. Steroid treatment also induced a leftward shift of the force-frequency curve at 25 and 50 Hz when compared with saline treatment (p < 0.05). ATPase staining of the diaphragm, scalenus medius, and gastrocnemius showed type IIb fiber atrophy in the steroid groups and also diaphragmatic type IIa atrophy with T, whereas histologic examinations revealed a normal muscular pattern with absence of necrosis. Finally, a pair-fed (PF) study, performed in 18 rats (C, T, and PF), showed that muscle atrophy was considerably less pronounced in PF animals than in T-treated animals. We conclude that (1) short-term treatment with massive doses of steroids induced severe respiratory and limb muscle wasting; (2) both types of steroids induced predominantly type IIb atrophy, resulting in the expected alterations in diaphragm contractile properties; (3) neither steroid caused muscle necrosis; (4) type IIb atrophy was not caused by acute nutritional deprivation alone.