In order to characterize the muscarinic binding site on coronary smooth muscle, we investigated the binding properties of (3H)quinuclidinyl benzilate (QNB) in membrane preparations of pig coronary arteries and atria. Scatchard analysis and Hill plot showed that (3H)QNB binds to a single population of sites in both tissues. The binding profiles of the muscarine receptor antagonists atropine, 11-[2-[dimethylamino)methyl)-1-piperidinyl)acetyl)-5,11-dihydro-6H-pyrid o(2,3- b) (1,4)benzodiazepine-6-one (AF-DX 116), pirenzepine, and 4-diphenylacetoxy-N-methylpiperidine methiobromide (4-DAMP) in both tissues were compared with binding data from other tissues, representative for different muscarinic binding site subtypes. It is concluded that the pig coronary smooth muscle muscarinic binding site is different from M1 and M2 binding sites investigated so far.