Sulfoconjugation and glucuronidation of salicylamide by isolated hepatocytes were examined with various concentrations of salicylamide and sodium sulfate. The ratio of sulfate to glucuronide formed changed markedly, depending on the concentrations of salicylamide and inorganic sulfate in the medium. The apparent Km value of sulfoconjugation for salicylamide was 0.006 mM, while its Vmax value varied depending on the concentration of inorganic sulfate (e.g., 2.1 and 0.5 nmoles/min/10(6) cells at 1.2 and 0.5 mM inorganic sulfate, respectively). The Km and Vmax values of glucuronidation for salicylamide were 0.19 mM and 1.28 nmoles/min/10(6) cells, respectively, in the absence of sodium sulfate. Glucuronidation was suppressed in the presence of inorganic sulfate. The suppression could be attributable to the competitive consumption of salicylamide by sulfotransferase. Additional in vivo experiments revealed that an extra amount of salicylamide markedly lowered the blood inorganic sulfate levels of rats. The significance of the finding is discussed in conjunction with the variation of the Vmax value of sulfoconjugation with the inorganic sulfate concentration.