The muscarinic acetylcholine receptor was solubilized from rat brain cortex by the zwitter-ionic detergent, 3-[3-cholamidopropyl)dimethylamino)-1-propane sulfonate (CHAPS). The supernatant, after centrifugation at 100,000 x g, was shown to contain molecules with binding sites for both 3H-pirenzepine (3H-PZ) and 3H-(-) quinuclidinyl benzilate (3H-QNB). Maximum binding values for 3H-PZ and 3H-QNB binding to solubilized receptors were approximately 176 +/- 24 pmol/g and 370 +/- 53 pmol/g of protein, respectively. The Kd values for 3H-PZ and 3H-QNB binding to solubilized receptors were 27 +/- 6.3 nM and 0.17 +/- 0.03 nM, respectively. The rank order of potencies of muscarinic drugs, in terms of their ability to inhibit binding of either 3H-PZ or 3H-QNB, was atropine greater than pirenzepine greater than oxotremorine greater than carabachol. Pirenzepine inhibited 3H-QNB binding with a Hill coefficient of 0.77, but inhibited 3H-PZ with a Hill coefficient of 0.94. Hill coefficients for agonists were less than 1. These findings indicate that muscarinic receptors solubilized from rat brain cortex retain their abilities to interact selectively with muscarinic receptor agonists and antagonists.