The regulation of purine metabolism in rat liver has been examined under conditions that alter the flux through the pathway. Rats were given intraperitoneal injections of ethanol, sodium acetate, or sodium phosphate to attain body water concentrations of approximately 70, 20, and 10 mM, respectively. The livers were freeze-clamped after 30 min, and extracts were made for the analysis of metabolites, cofactors, purine bases, and nucleosides; homogenates were made for the measurement of the activities and kinetic parameters of seven enzymes that participate in purine salvage. The values of the equilibrium constants of nine reactions were determined in vitro and compared with the ratios of the reactants measured in liver. The changes in phosphoribosylpyrophosphate (PRPP), a key intermediate in both the de novo and salvage pathways of purine metabolism, were directly correlated with the changes in ribose 5-phosphate (ribose-5-P); ([PRPP] = 1.7[ribose-5-P] - 7.4 mumol/kg). Ribose-5-P concentrations in turn could be predicted from the liver content of fructose 6-phosphate and glyceraldehyde 3-phosphate by calculation from the known equilibria. The maximum velocities in the tissue of the seven enzymes measured were calculated from the measured substrate values in the liver and with consideration of other effectors of enzyme activity. PRPP synthetase was the least active of the enzymes measured, indicating a possible rate-limiting step. The delta G of the enzyme steps differed from equilibrium values by factors ranging from 4 (nucleoside phosphorylase) to 10(5) (PRPP synthetase and purine transferase reactions). The regulation of purine salvage appeared to depend on the levels of PRPP and ribose-5-P.