Single cells were prepared from guinea pig tracheal smooth muscle and used in binding studies of [3H]p-aminoclonidine. Specific binding of [3H]p-aminoclonidine was saturable to a single class of receptors, with an equilibrium dissociation constant (KD) of 1.62 +/- 0.17 nM and a density (Bmax) of 6.20 +/- 0.78 x 10(4) sites/cell. Competition studies were carried out with several adrenergic antagonists. The rank order of potency was idazoxan = phentolamine > yohimbine > prazosin = corynanthine. These results suggest that the alpha 2-adrenoceptor in guinea pig tracheal smooth muscle represents a single pharmacological subtype, which is designated as alpha 2D.