The effects of storage at 4 degrees C on the antagonist and agonist binding properties of the muscarinic acetylcholine receptor from fresh surgical and frozen autopsy samples from human cerebral cortex were studied. The number of L-[3H]3-quinuclidinyl benzilate binding sites and their affinities were stable up to 51 h, both when stored as pieces of intact nonfrozen tissue and as a homogenate. The agonist binding properties as measured by the ability of the muscarinic agonist carbachol to compete with L-[3H]3-quinuclidinyl benzilate were also stable up to 51 h when the tissue was stored in the form of pieces. The affinity for carbachol decreased when the tissue was stored as a homogenate. The frozen autopsy samples showed no significant differences in binding properties in comparison with fresh neurosurgical tissue.