Cyanosis in a 36 year old patient which persisted 20 years after a successful surgical closure of her patent foramen ovale has been finally diagnosed as due to congenital methemoglobinemia: a 28% level of methaemoglobin and no activity od NAD-dependent methaemoglobin reductase were found. Erythrocyte antioxidative system was studied i.e. glutathione peroxidase, reductase, transferase, superoxidase dismutase and glucose-6-phosphate dehydrogenase. Increased activity was disclosed of superoxide dismutase and glucose-6-phosphate dehydrogenase in erythrocytes in comparison to normal individuals as well as increased concentration of lipid peroxidation products coexisting with methaemoglobin reductase deficiency.