A sensitive and selective method of determination of N,N-dipropylamino-5, 6-dihydroxytetralin and its 6,7-dihydroxy isomer in rat plasma and brain tissue is described. The method is based on isolation of these dopamine receptor agonists from the biological samples on small Sephadex G-10 columns, followed by reversed phase high performance liquid chromatography with amperometric detection. Large volume (2 ml) injections result in on-column sample enrichment without a subsequent loss in column efficiency. Using this method the very low detection limit of 4 pmol/g was achieved in brain tissue. It was found that the lower in vivo potency as a centrally acting dopamine agonist of the 6,7-compared with the 5,6-isomer is almost entirely due to the lower brain concentrations achieved after peripheral administration of equimolar doses. This difference is mainly caused by different rates of O-methylation by catechol-O-methyltransferase, which follows from the fact that inhibition of this enzyme by tropolone leads to almost equal brain concentrations of the two isomers.