After infusion of sodium D(-)L(+)-lactate in healthy man the clearance of the D(-)-isomer from blood was 70% of that of the L(+)-isomer. Utilisation of L(+)-lactate may have been inhibited by the presence of the D(-)-isomer. The changes in blood pyruvate concentration and ketone body ratio were compatible with mitochondrial oxidation of D(-)- lactate to pyruvate. After infusion of a D(-)L(+)-lactate racemic mixture, the renal excretion of the D(-)-isomer was much greater than that of the L(+)-isomer, although peak blood concentrations of the L(+) were higher than those of the D(-)-isomer. After infusion of sodium L(+)-lactate, the renal excretion of D(-)-lactate increased 7 times, although no D(-)-lactate could be detected in the blood or in the sodium L(+)-lactate infused.