Sulfation is the conjugation of chemicals with sulfate, which usually decreases, but occasionally increases, their biological effects. The phenol-sulfotransferase inhibitor pentachlorophenol (PCP) is often used to distinguish the biological effects of a chemical from its sulfate conjugate. Recently, molybdate has been shown to decrease the hepatic concentration of 3'-phosphoadenosine 5'-phosphosulfate (PAPS), the cosubstrate for sulfation. Therefore, the present study was designed to compare the effectiveness and specificity of molybdate and PCP as inhibitors of sulfation. Alpha-naphthol (125 and 250 micromol/kg, i.p.) was administered to rats and the sulfate and glucuronide conjugates excreted into urine were quantified for this comparison. Molybdate (5.0, 7.5, and 10 mmol/kg) decreased the 24-h cumulative urinary excretion of the sulfate conjugate of the lower dose of alpha-naphthol by 54, 53, and 55%, respectively, with corresponding compensatory increases in glucuronide excretion at the two lower doses of molybdate. PCP (20, 40, and 80 micromol/kg) similarly decreased the sulfation of alpha-naphthol by 48, 38, and 41%, respectively, but prevented compensatory increases in glucuronide excretion. Molybdate (2.5, 5.0, and 7.5 mmol/kg) decreased the sulfation of the higher dose of alpha-naphthol by 21, 30, and 44%, respectively, again with corresponding compensatory increases in glucuronide excretion. In contrast, PCP did not decrease significantly the sulfation of the higher dose of alpha-naphthol. These data suggest that molybdate is equally or more effective than PCP at inhibiting sulfation of alpha-naphthol, and appears to be more specific.