It is generally accepted that full and partial agonists interact with the same receptors according to the classical receptor mechanisms. We have modified the drug receptor mechanisms of M3-, alpha 1- and beta-receptors. Among the muscarinic receptors, there are two subtypes of M3-cholinoceptors, propylbenzilylcholine mustard (PrBCM)-sensitive receptors and (PrBCM)-resistant ones. Full agonists contract the guinea pig ileum through both types of cholinoceptors, while the partial agonists produce contractions through only the PrBCM-sensitive receptors. Two subtypes of alpha 1-adrenoceptors, alpha 1A and alpha 1B, were demonstrated in some arteries. Full agonists contracted the rabbit aorta through both the alpha 1A- and alpha 1B-adrenoceptors, while the partial agonists mediated contraction through only the alpha 1A-adrenoceptors. beta-Chloroethylamines (PrBCM and chloroethylclonidine) can discriminate the subtype of M3- or alpha 1-receptors in the presence of GTP. beta-Adrenoceptors have two different types of binding sites, high and low affinity sites. The competitive antagonistic effect of the partial agonist is due to their ability to compete with the full agonists for the high affinity site, while the partial agonists interact with the low affinity site to induce the beta-adrenergic effect. A regional difference in alpha 1-adrenoceptor mechanisms was discussed. The potency of norepinephrine in veins is related to alpha 1-adrenoceptor densities. In contrast, the potency of norepinephrine is linearly related to the agonist dissociation constant. This discrepancy suggests a qualitative difference between alpha 1-adrenoceptor mechanisms in the veins and arteries.