The p-nitrophenol (PNP) sulfate conjugation rate in rat liver cytosol at pH 7.4 under 10-30 microM 3'-phosphoadenosine-5'-phosphosulfate (PAPS) was decreased at 10-200 microM PNP and increased again at more than 200 microM PNP. The higher the PAPS concentration, the more remarkable such substrate inhibition. These results indicate that the substrate inhibition was due to the direct interaction of PNP with the aryl sulfotransferase (phenol sulfotransferase, PST). The PST reaction was thermolabile at less than 20 microM PNP but thermostable at more than 200 microM PNP. The Km values for PAPS obtained in the PST reactions where the inhibition was not observed was about 23 microM at 2.5-5 microM PNP and 11-14 microM at more than 200 microM PNP. And the Km values for PNP at more than 500 microM PNP was 2.2-2.6 mM. On the other hand, the inhibition in the PST reaction at pH 5.6 was observed at much higher PNP concentration, approximately 200 microM, than that at pH 7.4. Based on these characteristic results, the correspondence of the PST catalyzing the PNP sulfate conjugation (pH 7.4) at the low and high concentrations of PNP in the liver cytosol with the multiplicity obtained in the purified preparation of the liver previously reported by another researchers was compared and discussed. Finally the selection of an endogeneous substrate for estimating such complexed sulfate conjugation in advance in each individual man was also proposed.