The extent of transfer of carbon atoms from alanine to glucose was examined in pups and grown dogs. [U-14C]Alanine and [2-3H]glucose were injected intravenously and by using the SAAM-26 program a compartmental model was formulated from the tracer data to quantify the kinetics of the alanine- and glucose-carbon system. A 3-compartment model was necessary to describe the alanine-carbon kinetics. Strict identification of these compartments with physiological counterparts was not possible. The overall transport (turnover) rate of alanine-C was found not to change significantly with age when calculated on the basis of body weight. Carbon atoms from alanine reach glucose by three identifiable pathways of different speed. The kinetics suggest that the different pathways are at least in part due to differences in the metabolism of the three carbon atoms of alanine. In pups less than 11 days of age 45-49% of the carbon atoms leaving the alanine subsystem enter glucose; in adults it was 70%. In young pups 18-19% of glucose-C utilized was derived from alanine-C, whereas in adults, 62%. Accordingly a smaller percentage of carbon atoms utilized as alanine appears in glucose and a smaller percentage of glucose-C is derived from alanine in pups than in grown dogs.