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The occurrence of spare receptors, and partial agonists for smooth muscle contractions mediated by Alpha 1-adrenergic receptors can be accounted for with a ternary complex model of drug action. In this model, receptor-ligand complexes are assumed to inactive until the complex binds to an activating protein. Contractile responses are assumed to be proportional to the concentration of ternary complex (receptor-ligand-activator) regardless of the ligand involved. Antagonists are unable to form the ternary complex. Spare receptors are present as the inactive receptor-ligand complex. Such a model is shown to fit already published data on membrane binding of alpha -adrenergic agonists as well as contractile responses to the agonist. The proposal that contractile responses are proportional to the ternary complex concentration could be tested by fitting the ternary complex model to the data from studies of contractions induced by partial agonists as well as full agonists.