The characteristics of atropine-sensitive binding of l-[3H]-quinuclidinyl benzilate ([3H]-QNB) to membrane re-suspensions of sheep posterior pituitary indicate that the binding sites represent muscarinic cholinergic receptors. Scat-chard plots of 6 saturation experiments showed a single class of binding sites, with an equilibrium dissociation constant of 16 ± 2 pM and a density equivalent to 1.8 ± 0.2 pmol/g wet weight of tissue. Kinetic analysis of 2 association and 3 dissociation curves yielded mean association and dissociation rate constants of 3.9 × 108M–1 min–1 and 4.3 × 10–3 min–1, respectively. The binding had a detailed pharmacology for 12 drugs consistent with muscarinic receptor identification. In rat neurointermediate lobes, superior cervical ganglionectomy had no demonstrable effect on [3H]-QNB binding. The location(s) and functional role(s) of neurohypophyseal muscarinic receptors remain to be