While structure-activity relationship studies suggest that one necessary criterion for high-affinity binding of ligands to benzodiazepine receptors (Bz R) is the ability of these molecules to assume a planar or pseudoplanar topography, to our knowledge no completely rigid compounds with a planar geometry have been shown to bind with high affinity to Bz R. We now report the synthesis of high-affinity ligands of the Bz R based on the 7,12-dihydropyrido[3,2-b:5,4-b]diindole system 2, which have been shown to possess high affinity (5-15 nM) for Bz R. Moreover, substitution on the E ring of the pyridodiindoles has a marked effect on both the in vitro binding affinity and the pharmacological activity of these compounds: replacement of either hydrogen or methoxyl at position 2 of 2 by a chlorine atom results in a change in the activity from an inverse agonist to an antagonist. These rigid, planar pyridodiindoles represent a new class of high-affinity ligands for Bz R. In vivo evaluation indicates that 2 and 4e behave as partial inverse agonists at the Bz R, while the 2-chloro analogue 4d is an antagonist that lacks inverse agonist activity. The significance of these results lies in the rigid, planar nature of these pyridodiindole ligands, which have no conformational freedom, bind with high affinity to Bz R, and exhibit different pharmacological profiles. Rigid analogues of these structures will be important for searching agents to selectively target Bz receptor subtypes and probing the pharmacophore structure of Bz R.