Formation of lactate and pyruvate from glucose was studied in cultured amniocytes and chorionic villus fibroblasts from controls, either untreated or treated with azide, an inhibitor of cytochrome c oxidase, or other inhibitors of the mitochondrial respiratory chain. Amniocytes with an established cytochrome c oxidase deficiency were also investigated. Control amniocytes treated with azide as well as cytochrome c oxidase deficient amniocytes displayed strongly increased lactate-to-pyruvate ratios after incubation with glucose, compared to control cells. Elevated lactate-to-pyruvate ratios were also found in chorionic villus fibroblasts in which complexes I, III or IV were inhibited by rotenone, antimycin or azide, respectively. We conclude that measurement of lactate and pyruvate production from glucose in cultured amniocytes and/or chorionic villus fibroblasts allows adequate prenatal diagnosis of systemic cytochrome c oxidase deficiency and presumably of other systemic deficiencies of mitochondrial respiratory chain enzymes.