1. A sodium L(+) lactate infusion technique is described. 2. The results were fitted satisfactorily to a single compartment model, from which the endogenous production rate, fractional rate removal constant and volume of distribution of lactate were derived. The fractional rate removal constant of lactate was assumed to be independent of the blood lactate level. 3. Obesity was associated with an elevated endogenous production rate of lactate but the rate of lactate removal was normal; the rate of lactate removal was impaired in subjects with maturity-onset diabetes mellitus. 4. The volume of distribution of lactate, as a percentage of body weight, was similar in control, obese and maturity-onset diabetic subjects, and was of the order of magnitude of the extracellular fluid volume. 5. The relation of these findings to blood pyruvate and lactate levels after oral glucose administration to subjects with obesity and maturity-onset diabetes is discussed.