Enzymatic and histological features of muscular disorders associated with primary aldosteronism and glycyrrhizine-induced pseudoaldosteronism were studied. Among 10 patients with primary aldosteronism and 3 patients with pseudoaldosteronism, 5 patients were admitted to our hospital because of muscular weakness. The serum potassium (K) level was 1.86 +/- 0.21 mEq/l in a myopathy group on admission, a value significantly less than that of the 2.74 +/- 0.10 mEq/l in a non-myopathy group (p less than 0.01). Serum creatine phosphokinase (CPK), glutamate-oxyloacetate transaminase (GOT), and lactate dehydrogenase (LDH) were increased in the myopathy group compared to the non-myopathy group; serum CPK was 1412.6 +/- 902.6 vs. 22.8 +/- 5.0 mU/ml, serum GOT was 186.4 +/- 75.3 vs. 24.2 +/- 5.4 mU/ml (p less than 0.05), and serum LDH was 1133.4 +/- 377.3 vs. 387.6 +/- 42.5 mU/ml (p less than 0.05) in the groups with and without myopathy. Analysis of CPK isozymes revealed that the MM type exceeded 95%. The elevated serum CPK, GOT and LDH rapidly decreased to the normal range and muscular strength completely improved within 6 to 13 days after hospitalization, when the serum K level remained below than normal. Light microscopic finding of damaged muscle showed the diffuse necrosis and vacuolization of muscle fibers. Electron microscopic study clearly demonstrated complete dissolution of myofilaments with disappearance of sarcoplasmic reticulum and T-tubules in the necrotic muscle fibers. These results indicate that muscular lesions may occur in primary aldosteronism and pseudoaldosteronism when the serum K level is decreased to below 2.0 mEq/l. This myopathy is not periodic paralysis but hypokalemic myopathy. The mechanism by which K deficiency causes muscular damage remains unknown.