Biochemical studies have been performed in muscle, liver, leukocytes, and fibroblasts from patients suffering from the Zellweger syndrome. Oxidation rates of [1-14C]pyruvate, [U-14C]malate, and [1-14C]2-oxoglutarate were strongly reduced in skeletal muscle homogenate. Oxygen consumption in isolated skeletal muscle mitochondria could only be stimulated by ADP in the presence of ascorbate and N,N,N1,N1-tetramethyl-p-phenylenediamine. Cytochrome contents in heart muscle and liver mitochondria were normal. A very low activity of succinate-ubiquinone oxidoreductase was found in liver homogenate of two patients. From the effect of 2-thenoyltrifluoroacetone on the succinate-phenazine methosulphate oxidoreductase activity, a nearly competitive inhibition with respect to phenazine methosulphate was demonstrated in contrast with a non-competitive inhibition in controls. Normal oxidation rate of [1-14C]pyruvate and [2-14C]pyruvate was found in leucocytes and fibroblasts. Lactate and pyruvate levels were normal in serum and cerebrospinal fluid and beta-hydroxybutyrate and acetoacetate levels were normal in blood. The ratios lactate/pyruvate and beta-hydroxybutyrate/acetoacetate were normal as well. These findings point to a defect in the electron transport chain at the succinate-ubiquinone oxidoreductase level. This defect might be related to the absence of peroxisomes in the cells of Zellweger patients.