Cholinergie agents arepotent modulators of insulin release that aet via musearinie reeeptors. We now investigated the musearlnie receptor subtype present in rat panereatic islets in binding and funetional studies. Binding of 5 nM [ 3 H]N-methylscopolamine ([ 3 H]NMS) was half maximal at 30 min. At 60 min, the maximal total bindingwas 1.29% and the non-specifie binding (presence of 100 ,uM atropine) was 0.18% of the total radioaetivity per 10 f.'g islet protein. The high-affinity Kds were 8.5, 56, 1300 and 1300 nM, respectively. The high affinity Kd of the muscarinie receptor agonist, arecaidine propargyl ester (APE), was 8.1 nM. The EC 50 for the biologieal effects of APE on insulin and glucagon secretion was 3.2 and 2.3 nM. The rank order for the high-affinity biological effects of antagonists (inhibition of APE-mediated insulin/ glucagon release) was almost the same as for binding. The data indicate that rat pancreatie islets contain neither an MI subtype (high-affinity for pirenzepine) nor an M 2 subtype (high-affinity for methoctramine) receptor. However, the data evidence an M 3 receptor subtype, since SiHC in the absence of the M 1 receptor subtype shows a relatively high affinity to the receptors in rat panereatie islets.