Patients with homocystinuria excrete a large amount of metal in their urine. Homocysteine similar to penicillamine, administration to methylmercury treated rats resulted in a large amount of urinary methylmercury excretion. These results suggested that the total metal amounts in the whole body of patients with homocystinuria might be decreased. However, actually metal concentrations in hair and plasma of these patients were higher than those of normal controls. High plasma and hair metal levels are not accounted for in patients with homocystinuria. The physiological metal excretory mechanism in which small amounts of metals bind to the small, plasma molecular substances filter through the kidney and emerge in the urine is necessary for reconfirmation. Strongly perturbed metal metabolism exists in the patients with homocystinuria.