On the basis of the benz[d]indolo[2,3-g]azecine derivative 1 (LE300), structure-activity relations were investigated in order to identify the pharmacophore in this new class of ligands. Various structural modifications were performed and the inhibitory activities at human cloned D(1), D(2L), and D(5) receptors were measured by using a simple fluorescence microplate reader based calcium assay. Subsequently, the affinities of active compounds were estimated by radioligand binding experiments. Deleting one of the aromatic rings as well as replacing it by a phenyl moiety abolishes the inhibitory activities almost completely. Contraction of the 10-membered central ring decreases them significantly. The replacement of indole by thiophene or N-methylpyrrole reduces the inhibitory activity, whereas replacing the indole by benzene increases it. Finally, the hydroxylated dibenz[d,g]azecine derivative 11d (LE404) was found to be more active than the lead 1 in the functional calcium assay as well as in radioligand displacement experiments.