To analyze muscarinic receptors on rat pancreatic acini, we studied the binding of 125I-quinuclidinyl benzilate (125I-QNB) and N-[3H]-methylscopolamine ([3H]-NMS) to these acini. Binding of 125I-QNB and [3H]-NMS to acini was specific and reversible. 125I-QNB bound to low affinity site, which was not recognised by [3H]-NMS. However, nonspecific binding of 125I-QNB to acini was very high (46%), so 125I-QNB may be inadequate to analyze muscarinic receptor on pancreatic acini. Muscarinic receptors are classified in two groups, M1 and M2, according to affinity of pirenzepine which binds to M1 receptor selectively. Pirenzepine was 530 times less potent than atropine in inhibiting the binding of 125I-QNB, and 250 times less potent than atropine in inhibiting the binding of [3H]-NMS. These results suggest that muscarinic receptors on pancreatic acini are mainly M2 receptors.