Studies were performed on anesthetized opossums. Lower esophageal sphincter pressures were measured with waterfilled and continously perfused catheters anchored in the sphincter. Administration of adenosine 5'-triphosphate (ATP) and ADP in the left gastric artery usually produced an initial contraction followed by a more prolonged inhibition. Adenosine, on the other hand, produced only inhibition. The effect of ATP was not modified by phentolamine (1 mg/kg), propranolol (1 mg/kg) or atropine (30 micrograms/kg). Tetrodotoxin partially antagonized (P less than 0.05) the inhibitory effects of ATP and adenosine, but augmented the excitatory effect of ATP (P less than 0.05). 2-2'-Pyridylisatogen tosylate selectively antagonized the effects of ATP without modifying the effect of adenosine. This antagonist, however, did not modify the resting sphincter pressure or the vagal stimulated sphincter relaxation. Dipyridamole enhanced the inhibitory effect of adenosine, but did not augment vagal stimulates sphincter relaxation. There was no cross-tachyphylaxis between ATP and adenosine. Tachyphylaxis, either to ATP or adenosine, did not inhibit the vagal stimulated sphincter relaxation. These studies show that: (a) ATP and adenosine may act on different receptor sites to modify sphincter pressure; (b) only a small part of the inhibitory effect of ATP and adenosine is mediated by the inhibitory neurons; (c) neither ATP nor adenosine appears to be the inhibitory transmitter released by the noncholinergic, nonadrenergic inhibitory neurons in the vagal pathway to the lower esophageal sphincter.