Structural determinants of the allosteric interaction of semi-rigid verapamil analogues with dihydropyridine binding were investigated in kitten heart using [3H](+)-isradipine as radioligand. Chemical variations were performed in the alkyl chain of verapamil and include introduction of unsaturation (double or triple bonds) or the insertion of cyclohexyl moieties. Introduction of unsaturation generally reduces the allosteric interaction in the case of 'double bond'-and abolishes it in the case of 'triple bond'-derivatives. Also the introduction of cyclohexyl moieties diminishes the potency of allosteric interaction: derivatives with the phenylethylamino side chain in an equatorial position exhibit the allosteric interaction, while it is lacking in derivatives with the basic side chain in axial position. Thus, the reduced conformational flexibility of the new verapamil congeners reduces or abolishes their ability to allosterically interfere with dihydropyridine binding. A molecular interpretation was approached by molecular modelling studies. The strategy was to find low energy conformations common to the active congeners, but not shared by the inactive ones. Structural features discriminating allosterically active and inactive congeners comprise: 1) the position of the nitrogen, 2) the volume occupied by the N-methyl groups, 3) the direction of the N-H bond and 4) the position of the phenyl ring in the basic side chain.