To elucidate whether increased plasma levels of free dopamine (F-DA) after exercise are due to deconjugation of sulfoconjugated (S-) DA in plasma, we compared the changes in plasma F- and S- DA, as well as changes in both the S- and F- forms of epinephrine (E) and norepinephrine (NE), after running a half-marathon. Free catecholamines (F-CAs) were measured by automated high-performance liquid chromatography (HPLC). Total (F+S) CAs were determined using an efficient deconjugation method as follows; 1200 microliters plasma was incubated with 152 mU arylsulfatase (AS) for 30 min at pH 7.6. The plasma levels of F-CA (pg/ml) (mean +/- SEM) all increased significantly (p < 0.01) after the half-marathon; i.e., F-DA increased from 13.3 +/- 5.7 to 176.3 +/- 32.2; F-E from 58.0 +/- 12.3 to 764.3 +/- 136.4; F-NE from 246.6 +/- 15.2 to 3082.0 +/- 690.3. OF S-CAs, S-E (from 127.8 +/- 26.0 to 1218.2 +/- 190.8) and S-NE (from 717.1 +/- 61.6 to 5586.9 +/- 761.9) also increased, but, in contrast, among the S-CAs, only the increase in S-DA (from 5324.9 +/- 1967.3 to 7359.6 +/- 1627.9) was not statistically significant. Sulfoconjugation may play an important role in inactivating F-DA as well as F-NE and -E, that are released into plasma in response to vigorous exercise. Thus, plasma F-DA is unlikely to be derived through deconjugation of plasma S-DA.