Novel ligands that bind irreversibly and selectively to "peripheral" type benzodiazepine receptors (PBR) have been prepared. These compounds inhibit radiolabeled binding to PBR in the nanomolar range. The 2-isothiocyanatoethyl analogue of Ro 5-4864 (1-methyl-7-chloro-1,3-dihydro-5-(4-chlorophenyl)-2H-1,4-benzodiazepi n-2-one) (5, AHN 086) was synthesized in three steps from desmethyl Ro 5-4864. The (+/-) (11a, AHN 070), R-(-) (11b), and S-(+) (11c) 2-isothiocyanatoethyl derivatives of PK 11195 (1-(2-chlorophenyl)-N-methyl-N-(1-methylpropyl)-3-isoquinoline-carboxami de) were each prepared in three steps from PK 11209 (1-(2-chlorophenyl)-3-isoquinolinecarboxylic acid, 6). All four compounds inhibited radioligand binding to the PBR in brain and kidney. The R-(-) stereoisomer 11b was observed to be approximately 2.5-fold more potent than its enantiomer 11c; this is the first report of stereoselectivity in the isoquinoline series of ligands selective for the PBR. Furthermore, pH dependency studies showed that, at lower pH, change in the affinities for the PBR ligands is a property of the receptor, substantiating the hypothesis that a histidine moiety on the PBR is the most likely site for covalent bond formation, whereas, at higher pH, the observed changes in affinities can be attributed to properties of the compounds. All four of these novel ligands are potentially useful tools in the investigation of the PBR.