A protocol of constant infusion of fructose has been carried out both in human volunteers and in the perfused rat liver, aiming at a steady-state blood fructose concentration of 6-8 mM. Localized 31P-NMR spectroscopy and biochemical analyses were used to evaluate the metabolic changes. Comparison of the model experiment and the clinical study allowed an evaluation of this protocol as a clinically relevant assessment of the metabolic function of the liver. The time course of change, as well as the quasi steady-state levels reached during fructose infusion, for phosphomonoesters (PME), ATP and inorganic phosphate (Pi) provided the following results: During fructose infusion, ATP and Pi reached a steady-state level of 74.0 ± 5.9 and 54.6 ± 3.3% of control respectively, in the human volunteers. The corresponding data in the rat liver was 71.3 ± 4.3 and 54.4 ± 4.3%. Hepatic clearance of fructose was 0.53 and 0.52 ml · gl liver-1 · min-1 for volunteers and rats, respectively. The time course of intracellular metabolite recovery after fructose could be approximated by a first order kinetic. The rate constants for PME and ATP change were similar during fructose infusion and recovery, while after the discontinuation of fructose infusion Pi increased with a rate constant significantly greater than during its fructose-induced depletion in human liver (P < 0.005). Thus, this relatively simple clinically applicable protocol seems to be verifiable in the well controlled perfused rat liver model, and it is argued that it may be useful in the clinical evaluation of the metabolic functional capacity of the human liver.