The kinetics of lactate and pyruvate (1 and 5 mM in each case) metabolism was studied in isolated dog renal tubules. Utilization of these two substrates and the production of glucose, pyruvate, or lactate, and alanine were determined. The rates of lactate and pyruvate utilization and of glucose production were constant during 60 min of incubation. Glucose production from pyruvate was less than that from lactate. Addition of albumin to the incubation medium greatly inhibited lactate and pyruvate utilization at both substrate concentrations. It stimulated, however, glucose production from 1 mM, but not 5 mM, lactate or pyruvate. These effects were found to be due to the presence of fatty acids in the albumin solution used. In the absence of fatty acids, glucose production represented 35 to 40% of lactate uptake, but represented less than 20% of pyruvate uptake. Fatty acids markedly enhanced the percentage of transformation of lactate and pyruvate into glucose, and that of pyruvate into lactate. Alanine represented 20% or less of lactate and pyruvate uptake. These results suggest that fatty acids have a regulatory influence on lactate and pyruvate dog kidney metabolism.