To measure the rate of gluconeogenesis in humans directly, one must administer and determine the specific activity or the enrichment in an intermediate in the gluconeogenic process and in the glucose formed, thus obtaining the fraction of the glucose formed by gluconeogenesis. By a separate determination of the rate of hepatic glucose production, the rate of gluconeogenesis can then be calculated. The closer the intermediate is to glucose-6-P, the more complete will be the measurement of the rate. Thus, if the intermediate is below the level of the triose phosphates, gluconeogenesis from glycerol will not be included in the estimate. Estimates of rates of gluconeogenesis from estimates of PEP enrichment or specific activity require a measure of the extent of exchange of label at the level of oxaloacetate. By using 14C or 13C labeled CO2 as the intermediate and estimating the relative rates of the reactions of the tricarboxylic acid cycle relative to gluconeogenesis from the distribution of 14C from [3-14]lactate in glutamine from the glutamine conjugate of phenylacetate, the enrichment or specific activity of PEP has been estimated. Correction must be made for the incorporation into the glutamine of 14CO2 formed from the [3-14C]lactate. Data support the validity of this approach toward estimating gluconeogenesis in NIDDM, but the approach is complex, time consuming and with uncertainties. Estimates that have been made using [2-14C] acetate are invalid because of the extensive metabolism of [2-14C]acetate in other than liver. Other approaches have promise, but technical problems may exist in their use and other problems, such as hepatic zonation and exchange reactions, may compromise their application.