7-(Aminosulfonyl)-1,2,3,4-tetrahydroisoquinoline (SK&F 29661, 1) is a potent inhibitor of the enzyme phenylethanolamine N-methyltransferase (PNMT, EC 2.1.1.28). In contrast to other inhibitors of PNMT, it is also highly selective toward PNMT in comparison with its affinity toward the alpha 2-adrenoceptor (PNMT Ki = 0.55 microM, alpha 2 Ki = 100 microM, selectivity [alpha 2 Ki/PNMT Ki] = 180). A diverse set of compounds was synthesized and evaluated to probe the role of the acidic hydrogen of the aminosulfonyl group of 1 in imparting this selectivity. Compounds were designed to investigate the effect on selectivity of the acidity of the NH group [the 7-N-methyl (compound 5) and 7-N-(p-chlorophenyl) (compound 4) derivatives of 1], the relative spatial position of the acidic hydrogen [7-(N-(methylsulfonyl)amino)-1,2,3,4-tetrahydroisoquinoline (6) and 7-((N-(methylsulfonyl)amino)methyl)-1,2,3,4-tetrahydroisoquinoline (8)], or the effect of the substitution of an acidic phenolic group for the aminosulfonyl moiety [1-(aminomethyl)-6-hydroxynaphthalene (23) and 8-hydroxy-1,2,3,4-tetrahydrobenz[h]isoquinoline (9)]. All of the compounds studied displayed lower affinity for PNMT than 1, and nine of the eleven compounds studied showed increased, rather than the desired decreased, affinity for the alpha 2-adrenoceptor. Specifically, compound 4, in which the aminosulfonyl NH group is more acidic than that in 1, showed greatly reduced selectivity on account of increased alpha 2-adrenoceptor affinity as compared to 1 (PNMT Ki = 2.6 microM, alpha 2 Ki = 6.3 microM, selectivity = 2.4). Compound 8, in which the acidic NH group is in the same region of space as that in 1, although the aminosulfonyl group is reversed with respect to the aromatic ring, showed poor PNMT affinity and modest alpha 2-adrenoceptor affinity (PNMT Ki = 330 microM, alpha 2 Ki = 18 microM, selectivity = 0.055). Compound 9, in which a phenolic group is in the same region of space as the acidic NH of 1, exhibited the best alpha 2-adrenoceptor affinity of any of the compounds studied (PNMT Ki = 0.98 microM, alpha 2 Ki = 0.078 microM, selectivity = 0.080). Results from this study suggest that the selectivity of 1 is not solely due to the presence of an acidic hydrogen on the 7-aminosulfonyl group of 1 but is likely also dependent on some other property (e.g. electron-withdrawing character) of the aminosulfonyl group.