The M1, M2 and M4 subtypes of mAChRs are the predominant receptors in the CNS. These receptors activate a multitude of signaling pathways important for modulating neuronal excitability, synaptic plasticity and feedback regulation of ACh release. In addition, novel functions mediated by mAChRs are currently being discovered. These studies are greatly facilitated by the recent development of subtype selective toxins and mice lacking individual mAChR genes. Studies in cell culture and the rodent brain demonstrate that mAChR internalization and intracellular trafficking is an important component of mAChR regulation. Characterizing mAChR intracellular trafficking could help facilitate the development of selective mAChR ligands. For example, a selective M1 agonist would cause a shift in the distribution of M1 from the cell surface to an intracellular distribution, while M2 and M4 would remain on the cell surface. Characterizing mAChR intracellular trafficking is also important for understanding the cellular mechanisms that regulate mAChR cell surface expression and signaling. Furthermore, intracellular trafficking has recently been demonstrated to play a role in the development of tolerance to drugs (Whistler et al., 1999; He et al., 2002). Because individual mAChR subtypes are novel targets for treatments of diseases such as Alzheimer's disease and schizophrenia, understanding the mechanisms that regulate mAChR signaling and intracellular trafficking following acute and chronic stimulation might lead to the development of rational strategies.