Steady-state contractile responses elicited by activation of the proportional 1-adrenergic and 5-HT2 receptors in isolated rat and rabbit aorta, respectively, were analyzed. Agonist dissociation constants (KA's) obtained by the method of partial irreversible receptor inactivation were compared to KA values determined by fits of the operational model of pharmacological agonism to single concentration response curves (CRCs). The observed nature of the KA estimates obtained with the Furchgott method for phenylephrine (PE) and oxymetazoline (OXY) at the proportional 1-adrenergic receptor and for 5-hydroxytryptamine (5-HT) at the 5-HT2 receptor in isolated rat aorta, and for PE and 5-HT at the proportional 1-adrenergic and 5-HT2 receptors, respectively, in isolated rabbit aorta, was consistent with the hypothesis that the density of membrane receptors is greatly in excess of the density of transducer proteins (i.e., [Ro] much greater than [To]) in these systems. Therefore, KA, efficacy and slope factor estimates were also obtained by computer fits of the operational model to single agonist CRCs in both rat and rabbit aorta, with the empirically determined tissue maximal response (Tmax) substituted for the theoretical parameter Em. In all cases, the mean pKA estimates obtained with the operational model closely approximated and were strongly correlated with the mean pKA estimates determined by the Furchgott method. These studies suggest that, at least in some vascular preparations, Tmax is a good estimate of Em, and moreover, that Em may be not only a specific characteristic of a given receptor-effector system as previously demonstrated by Black and Leff, but that Em may also describe a more general feature of tissue responsiveness that is shared among distinct membrane receptors coupled to similar effector systems. In conclusion, when receptor inactivation studies have indicated that the condition [Ro] much greater than [To] exists, Tmax can be substituted into the operational model to provide valid estimates of agonist KA values at distinct receptor subtypes, in the absence of receptor alkylation.