Background: Sustained agonist-promoted ubiquitination of β-arrestin has been correlated with increased stability of the GPCR - β-arrestin complex. Moreover, abrogation of β-arrestin ubiquitination has been reported to inhibit receptor internalization with minimal effects on receptor degradation. Results: Herein we report that agonist activation of M1 mAChRs produces a sustained β-arrestin ubiquitination but no stable co-localization with β-arrestin. In contrast, sustained ubiquitination of β-arrestin by activation of M2 mAChRs does result in stable co-localization between the M2 mAChR and β-arrestin. Internalization of receptors was unaffected by proteasome inhibitors, but down-regulation was significantly reduced, suggesting a role for the ubiquitination machinery in promoting down-regulation of the receptors. Given the ubiquitination status of β-arrestin following agonist treatment, we sought to determine the effects of β-arrestin ubiquitination on M1 and M2 mAChR down-regulation. A constitutively ubiquitinated β-arrestin 2 chimera in which ubiquitin is fused to the C-terminus of β-arrestin 2 (YFP-β-arrestin 2-Ub) significantly increased agonist-promoted down-regulation of both M1 and M2 mAChRs, with the effect substantially higher on the M2 mAChR. Based on this observation, we were interested in examining the effects of disruption of potential ubiquitination sites in the β-arrestin sequence on receptor down-regulation. Agonist-promoted internalization of the M2 mAChR was not affected by expression of β-arrestin lysine mutants lacking putative ubiquitination sites, β-arrestin 2K18R, K107R, K108R, K207R, K296R, while down-regulation and stable co-localiztion of the receptor with this β-arrestin lysine mutant were significantly reduced. Interestingly, expression of β-arrestin 2K18R, K107R, K108R, K207R, K296R increased the agonist-promoted down-regulation of the M1 mAChR but did not result in a stable co-localiztion of the receptor with this β-arrestin lysine mutant. Conclusion: These findings indicate that ubiquitination of β-arrestin has a distinct role in the differential trafficking and degradation of M1 and M2 mAChRs.