Dichloroacetate is known to reduce plasma glucose and triglycerides in diabetic and starved animals and to lower plasma lactate under various experimental conditions. To investigate its metabolic effects in man, we administered oral doses (3 to 4 g) of dichloroacetate as the sodium salt to patients with diabetes mellitus or hyperlipoproteinemia or both for six to seven days. Dichloroacetate significantly reduced fasting hyperglycemia an average of 24 per cent (P less than 0.01) from base line and produced marked, concomitant falls in plasma lactate (73 per cent; P less than 0.05 to less than 0.01) and alanine (82 per cent; P less than 0.01 to less than 0.001). In addition, it significantly decreased plasma cholesterol (22 per cent; P less than 0.01 to less than 0.001) and triglyceride (61 per cent; P less than 0.01) levels while increasing (71 per cent; P less than 0.01) plasma ketone-body concentrations. Plasma insulin, free fatty acid and glycerol levels were not affected. Serum uric acid rose, whereas excretion and renal clearance fell. Some patients experienced mild sedation, but no other laboratory or clinical evidence of adverse effects was noted during or immediately after the treatment phase.