Binding studies in several species have demonstrated a high proportion of M1 muscarinic receptors in the lung but their localization is uncertain. Using [3H]quinuclidinyl benzylate we have confirmed that binding sites with high affinity for pirenzepine account for 50% of muscarinic receptors in the rat lung. Our functional studies using the muscarinic antagonists 4-diphenylacetoxy-N-methylpiperidine (4-DAMP), methoctramine and pirenzepine have demonstrated that the muscarinic receptor on the rat pulmonary artery endothelium which mediates vasodilation is of the M3 subtype and cannot account for the high proportion of M1 receptors identified in lung homogenates.