In opossum esophagus the highest density of varicose substance P-like immunoreactive deposits was in the muscularis mucosa, and this muscle layer, which is thick and prominent, was the most sensitive in contracting to exogenous substance P. On field stimulation, a tetrodotoxin-sensitive biphasic contraction was produced; an initial phasic component was followed after cessation of stimulation by a prolonged tonic component. This contractile response was unaffected by guanethidine or antagonists to several nerve mediators and to histamine and serotonin antagonists. Each component showed a different frequency dependence. The phasic component was abolished by atropine and potentiated by physostigmine; the tonic component was reduced or abolished after carbachol or physostigmine and potentiated by atropine. The tonic component was inhibited after substance P tachyphylaxis or by the substance P antagonist [D-Pro2, D-Trp7,9]substance P. On electron microscopic examination, the majority of nerve varicosities contained a mixture of small agranular and large granular vesicles. We postulate that acetylcholine and substance P may coexist in and be coreleased from nerves of this muscle and that the acetylcholine release occurs first, causing an initial phasic response and delaying and diminishing the release of substance P. The two mediators cause muscle contraction by actions on independent receptors.