The receptor reserves for acetylcholine, 5-hydroxytryptamine, and histamine in canine tracheal muscle were evaluated. Muscle strips were dissected free of epithelial and connective tissue and suspended for isometric tension recording in modified Krebs-Ringer solution. Dissociation constants for all three agonists were determined by analysis of their concentration-response curves under control conditions and after partial inactivation of receptors by phenoxybenzamine dihydrochloride. The values of KA for acetylcholine, 5-hydroxytryptamine, and histamine were 1.8 X 10(-5) M, 1.35 X 10(-6) M, and 5.0 X 10(-5) M, respectively. Dissociation constants were used to determine receptor occupancy-response relationships. Maximal responses to acetylcholine were obtained by activation of only 4.0 +/- 1.0% of receptors, indicating the presence of a very large receptor-reserve. In contrast, a maximal response to 5-hydroxytryptamine and histamine required activation of 78.0 +/- 11.0 and 87.7 +/- 1.6% of the receptors, respectively, indicating very modest receptor reserves. The differences in receptor-reserve characteristics for these agonists in airway muscle might contribute to the differential effects of inhibitory and facilitory influences on contractions elicited by them.