Spontaneous thymoma rats, Buffalo/Mna (B/Mna), in which nephrotic syndrom (NS) has recently been observed, have notable features in connection with muscle diseases ; they exhibit muscle fatigability and weakness. Some biochemical measurements used for diagnosis of muscle diseases and NS were performed in these rats. ACI strain served as a reference strain. Urinary creatinine level and serum enzyme activities such as CPK, aldolase, GOT and GPT in the B/Mna rats did not differ from those in the ACI rats. On the other hand, urinary creatine level, the ratio of urinary creatine to creatinine and serum total cholesterol level in the B/Mna rats were significantly greater than those in the ACI rats. B/Mna rats also showed proteinuria and hypoalbuminemia. These results indicate the possibility of some pathological change of skeletal muscles which may result at least partially from abnormal lipid metabolism and hypoproteinemia as a consequence of NS, differing from the typical muscular dystrophy.