1. Angiotensin-induced contraction of smooth muscle is accompanied by both homotropic (receptor-receptor) and heterotropic (receptor-G protein) cooperativity. 2. Binding constants for angiotensins II and III at uterine smooth muscle receptors have been compared in bioassays and binding assays, using the competitive antagonist Sarmesin to verify the binding assay/bioassay interrelationship. 3. Agonist affinities determined from binding studies in the presence of GTP/S were found to be similar to the affinities observed in responding rat uterine tissues under conditions which eliminate positive homotropic cooperativity, suggesting that heterotropic cooperativity and homotropic cooperativity are interdependent events for smooth muscle contraction. 4. The data are consistent with an allosteric or autosteric mechanism of receptor function involving cooperativity between two agonist binding sites on the receptor. 5. The model has been used to calculate homotropic efficacies for angiotensins II and III from bioassay data and binding data, respectively.