SUMMARY: Muscarinic receptors present in guinea-pig lung were characterized using the (M_{3}) selective radioligand (\left[{ }^{3} \mathrm{H}\right] 4)-diphenylacetoxy- (\mathrm{N})-methyl-piperidine methiodide (\left(\left[{ }^{3} \mathrm{H}\right] 4\right.)-DAMP). In saturation studies, (\left[{ }^{3} \mathrm{H}\right] 4) DAMP identified two populations of binding sites with approximately (4 %) of the sites displaying high affinity (\left(K_{d}=0.21 \mathrm{nM}\right.) and (B_{\max }=10 \mathrm{fmol} / \mathrm{mg}) prot.) while the remaining sites were low affinity ones ( (K_{\mathrm{d}}=18.11 \mathrm{nM}) and (B_{\max }=269 \mathrm{fmol} / \mathrm{mg}) prot.). In competition studies with [ (\left.{ }^{3} \mathrm{H}\right] 4)-DAMP ((0.35 \mathrm{nM})), methoctramine and hexahydro-siladifenidol (HHSiD) identified 50 and (70 %) of high affinity binding sites displaying the pharmacological profile of the (M_{2}) and the (M_{3}) receptors, respectively. No evidence was found for high affinity (\left[{ }^{3} \mathrm{H}\right]) pirenzepine binding sites in guinea-pig lung. However, pirenzepine/[3] ({ }^{3} \mathrm{H}) 4-DAMP competition experiments suggested that pirenzepine recognized an equal proportion of [3/4-DAMP binding sites with intermediate and low affinity binding constants. The intermediate affinity binding constant was inconsistent with the presence of (M_{1}) receptors and reflected more the presence of (M_{4}) or a mixture of (M_{3}) and (M_{4}) receptors. The low affinity pirenzepine binding sites may represent (M_{2}) receptors. These results provide further evidence for the occurrence of (M_{2}) and (M_{3}) receptors and suggest the presence of the (M_{4}) muscarinic receptor subtype in guinea-pig lung.