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Muscarinic cholinergic receptor sites in human gastric mucosa were analyzed directly by using radioligand binding techniques with the specific muscarinic antagonist 3H-quinuclidinyl benzilate (QNB) as ligand. Specific binding of 3H-QNB to membrane preparations from human gastric mucosa was saturable, of high affinity (Kd = 4.17 +/- 1.94 nM, Bmax = 0.37 +/- 0.04 pmol/mg protein) and selectively inhibited by muscarinic antagonists (atropine, scopolamine) and agonists (acetylcholine, pilocarpine). These findings provide direct evidence for the existence of muscarinic cholinergic receptors in human gastric mucosa. The specific 3H-QNB binding to its receptor was blocked by atropine but not by histamine, cimetidine, pentagastrin, or synthetic human gastrin. The muscarine and histamine H2-receptor, or muscarine and gastrin receptor, probably do not share the same locus.