To clarify the effect of subacute manganese feeding on serotonin and mineral metabolism, Wistar rats were separated into two groups and fed two different diets, one a normal diet and the another a manganese-supplemented diet. After three weeks on these dietary regimens, the rats on the manganese-supplemented diet manifested the following abnormalities: blood pressure was decreased; brain serotonin was decreased; L-aromatic amino acid decarboxylase activity in brain was decreased. Manganese levels in heart, lung, and kidney increased, whereas sodium, potassium, magnesium and calcium levels in the brainstem decreased.