To examine the effect of pravastatin administration on striated muscle ultrastructure, 10 BalbC mice were given pravastatin 40 mg/kg/day for 3 weeks. At the end of the study, blood was withdrawn for evaluation of the serum creatine phospho-kinase (CPK) level and the muscles of the hind legs, as well as the heart and liver of the animals were examined with a light and transmission electron microscope. After treatment with pravastatin the results showed a 101% increase in serum CPK level in comparison to untreated controls. Hematoxillin-eosin stained tissues of pravastatin treated mice did not show any abnormal findings. While the ultrastructure of the heart and liver of the treated animals appeared normal, the muscle fibers showed a marked alterations of the mitochondria, which were increased in size compared to those of the controls. The cristae were heavily damaged and even completely destructed, giving the mitochondria appearance of empty vacuoles. The findings are in favor of a specificity of pravastatin for striated muscles.