In humans and dogs, the plasma contains little free dopamine, and dopamine is almost all present in two isomeric forms of conjugates with sulfate esters, dopamine-3-O-sulfate and dopamine 4-O-sulfate. These two isomers differ in metabolic stability and biological activity. The physiological role of dopamine sulfates is controversial. In the present study on dogs, noradrenaline, dopamine and the two dopamine sulfate isomers in the peripheral arteries were measured by high-performance liquid chromatography, and the possibility that dopamine sulfate isomers are deconjugated in the arteries as a source of free dopamine was examined. The arteries were found to contain free dopamine and dopamine 3-O-sulfate at concentrations of 0.09-0.54 and 0.008-0.015 pmol/mg wet tissue, respectively. No dopamine 4-O-sulfate was found in the arteries or the plasma. Arylsulfatase activity was assayed by incubating a crude extract of arterial homogenate with dopamine 3-O-sulfate or dopamine 4-O-sulfate and measuring the dopamine produced. Exogenous dopamine 4-O-sulfate was desulfated by the extract, but dopamine 3-O-sulfate, which is the predominant isomer in the artery, was not desulfated by the extract. These results suggest that dopamine sulfate is not converted to dopamine in physiological conditions in the dog.