Addition of 5-HT or SK&F 103829 (2,3,4,5 tetrahydro-8[methyl-sulfonyl]-1 H-3-benzazepin-7-ol hydrobromide) contracts isolated strips of canine lower esophageal sphincter (LES) circular smooth muscle. 5-HT acts directly on the smooth muscle, since pretreatment with the neurotoxin TTX does not inhibit this contraction. Depletion of extracellular calcium or pretreatment with nifedipine inhibited the contraction to both 5-HT and SK&F 103829. Therefore, in this smooth muscle, the contraction produced by both 5-HT and SK&F 103829 requires extracellular calcium and is sensitive to inhibition by a voltage-dependent Ca2+ channel antagonist. In addition, with respect to 5-HT, SK&F 103829 appeared to act as a partial agonist. Receptor alkylation studies using phenoxybenzamine demonstrated no receptor reserve for the contractile response to 5-HT. Nonsurmountable antagonism of the contraction induced by 5-HT and SK&F 103289 was observed with several 5-HT2 antagonists, i.e., methysergide, ketanserin, cyproheptadine, and LY 53857. Using a method established for pseudoirreversible antagonism, the Ki values for these 5-HT2 receptor antagonists were estimated. Results suggested that both 5-HT and SK&F 103829 contract the canine LES by interacting at the same receptor site and that this receptor site has characteristics of the 5-HT2 receptor. Finally, neither bulbocapnine, domperidone, nor prazosin significantly alters the response to 5-HT or SK&F 103829. Thus, isolated strips of canine LES contain a contractile 5-HT2 receptor, and SK&F 103829 behaves as a partial agonist at this site.