Displacement of muscarinic radioligands by the cholinesterase inhibitors parathion, paraoxon, physostigmine and phenyl saligenin cyclic phosphate was examined in rat cortex and brain stem, human cortex and brain stem, and in Chinese hamster ovary (CHO) cells expressing human M2 or M4 muscarinic acetylcholine receptors. None of the cholinesterase inhibitors tested significantly affected binding of the antagonist [3H]quinuclinidyl benzilate. However, the agonist [3H]oxotremorine-methiodide (3H]oxo-M) was displaced by all compounds tested in a differential manner. Parathion only marginally displaced [H]oxo-M binding with pKi values < 5 in all tissue or cell types. In rat brain paraoxon, physostigmine and phenyl saligenin cyclic phosphate displaced [3H]oxo-M with pKi values of 7.5, 7.0 and 6.1, respectively. The cholinesterase inhibitors displaced [3H]oxo-M in human brain at 15- to 250-fold higher concentrations, that is with pKi values of 6.3, 4.6 and 4.2, respectively. Maximal displacement of [3H]oxo-M varied between 25% and 95%, depending on the species and the compound. Human receptors in brain and in CHO cells were equally sensitive to displacement of [3H]oxo-M by parathion, physostigmine and phenyl saligenin cyclic phosphate. However, paraoxon displaced [3H]oxo-M at > or = 35-fold lower concentrations from human receptors in brain than in CHO cells. In conclusion, the data show that cholinesterase inhibitors interfere with agonist binding to muscarinic acetylcholine receptors. The species-selectivity of the displacement appears to result from differences between rat and human muscarinic acetylcholine receptors. In addition, for paraoxon marked differences exist between the sensitivity of human muscarinic acetylcholine receptors in brain tissue and of those expressed in clonal CHO cells.