Corticosteroid myopathy was studied in young, mature New Zealand white rabbits given daily injections of betamethasone (0.3 mg/kg body weight/day) for two weeks. Control rabbits were pair‐fed and received saline injections. Betamethasone treatment caused significant wasting of type 2 gluteus medius and psoas muscles but did not cause any atrophy of type 1 soleus and gluteus minimus muscles. The Mg2+‐ and Ca2+‐activated myofibrillar ATPase activities of the corticosteroid‐treated rabbits did not differ from controls despite a 30% reduction in muscle wet weight and pronounced reduction in cross‐sectional area of fibers. SDS‐polyacrylamide gel electrophoresis profiles of myofibrillar proteins did not differ quantitatively or qualitatively between experimental and control rabbits. Studies of net muscle protein degradation (using 3H‐leucine) in betamethasone‐treated and control rabbits indicate that both type 1 and type 2 muscle fiber proteins are degraded several times faster in the corticosteroid‐treated group. This suggests that a compensatory mechanism exists for those type 1 and mixed fiber type muscles which have increased degradation but do not undergo wasting.