Chinese hamster ovary (CHO) cells are commonly used for expression of the genes of cloned neurotransmitter receptors to study their pharmacology and coupling to signal transduction pathways. It is usually assumed that host cells do not endogenously express the specific receptor under consideration. We demonstrate in this report that CHO cells contain endogenous functional muscarinic acetylcholine receptors which, in some circumstances, might complicate interpretation of data related to the properties of exogenously expressed receptors.