An enantiospecific method was developed for the synthesis of 3-fluoromethyl-, 3-hydroxymethyl-, and 3-chloromethyl-7-substituted-1,2,3,4-tetrahydroisoquinolines (THIQs) from phenylalanine. Biochemical evaluation of the enantiomers of these compounds at both PNMT and the alpha(2)-adrenoceptor indicates that both sites display similar stereoselectivity. Overall the R-enantiomer was usually the more potent enantiomer at both PNMT and the alpha(2)-adrenoceptor for these 3-fluoromethyl-, 3-hydroxymethyl-, and 3-chloromethyl-THIQs. The one exception is 3-hydroxymethyl-7-nitro-THIQ (9), which was found to display the opposite stereoselectivity at the alpha(2)-adrenoceptor. A comparison of the PNMT inhibitory potency of the enantiomers of these 3-fluoromethyl-, 3-hydroxymethyl-, and 3-chloromethyl-THIQs indicates that all of the 3-substituted-THIQs displayed similar inhibitory potency for PNMT. However, the nature of the 3-substituent was found to have a major effect on the alpha(2)-adrenoceptor affinity of these compounds with the 3-hydroxymethyl- and 3-fluoromethyl-THIQs having the highest affinity and THIQs containing the 3-chloromethyl moiety the least. Compounds R-3-fluoromethyl-7-cyano-THIQ (R-12) and R-3-fluoromethyl-7-N-(4-chlorophenyl)aminosulfonyl-THIQ (R-13) and both enantiomers of 3-chloromethyl-7-nitro-THIQ (R- and S-30) are the most selective inhibitors in this study and display selectivities (alpha(2)-adrenoceptor K(i)/PNMT K(i)) greater than 200. These compounds give important insight into the steric and stereochemical preferences of both PNMT and the alpha(2)-adrenoceptor, which should assist in the development of new PNMT inhibitors.