Muscarinic receptors in human tracheal smooth muscle were characterized by radioligand binding and functional studies. Specific [3H]-(-)-quinuclidinylbenzilate ([3H]-(-)-QNB) binding to tracheal smooth muscle membranes was reversible, stereoselective and of high affinity (Kd = 47 +/- 4 pmol/l; RT = 920 +/- 120 fmol/g tissue). Inhibition of specific [3H]-(-)-QNB binding by the M-1 selective antagonist pirenzepine was found to occur at relative high concentrations classifying the muscarinic receptor population as belonging to the M-2 subclass. Inhibition of specific [3H]-(-)-QNB binding by muscarinic agonists revealed the presence of high and low affinity sites in nearly equal proportions. 5'-Guanylylimidodiphosphate converted high affinity sites into low affinity sites although its effect was minimal. Log dose-contraction curves of methacholine had Hill coefficients of 1.10 +/- 0.04 with pD2-values of 6.75 +/- 0.02. Inhibition of specific [3H]-(-)-QNB binding by methacholine, however, was best described by a two binding site model with pKi-values considerably lower. The difference between these affinity values points to the presence of substantial receptor reserve.