Two patients, one dying at 25 days and one at 20 months had 'chronic' lactic acidaemia with a high lactate to pyruvate ratio. Both showed EEG abnormalities and seizure activity and both died of respiratory failure. Investigation of cultured skin fibroblasts from these patients revealed normal pyruvate dehydrogenase and pyruvate carboxylase activities but the cells showed a decreased ability to oxidase pyruvate which was returned to normal on the addition of methylene blue. Subsequent investigations revealed that the mitochondria from the patients' cells could oxidase pyruvate normally but that the cells had an abnormal NAD to NADH ratio under standard conditions of incubation. It was concluded that both children had a redox disequilibrium in the cytoplasmic compartment due to a problem in transporting reducing equivalents from the cytoplasmic to the mitochondrial compartments.