The in-vitro selectivity of a group of substituted benzamide drugs for brain neurotransmitter receptors was determined to assess the most appropriate drugs for use in human PET studies. All substituted benzamide drugs studied inhibited [3H]haloperidol and [3H]spiperone binding to rat striatal membranes. The most potent compounds were YM 09151-2, clebopride and raclopride. However, these substances also interacted in differing degrees with alpha-1, alpha-2, beta-adrenergic, 5-HT-1, 5-HT-2, and opiate sites. Sulpiride, alizapride, SL 74205, TER 1546 and tiapride were specific for D-2 receptors, but these drugs were active only in the 10(-7)-10(-6) M range. Raclopride, amisulpiride and sultopride showed a 100-1000 differentiation between action on dopamine sites compared with other neurotransmitter receptors. No such selectivity was observed for clebopride or YM 09151-2. Specific substituted benzamides such as alizapride, may be appropriate in high concentrations for defining the interaction of PET ligands with brain dopamine receptors. More potent, but selective, drugs such as raclopride and amisulpiride, may be effective in low concentrations as ligands for labelling dopamine receptor sites. However, the ability of these various substituted benzamide drugs to penetrate into brain and in-vivo to identify dopamine receptors in all brain areas must be assessed.