Chronic activation of the μ-opioid receptor (MOR1TAG) results in the loss of agonist response that has been attributed to desensitization and down-regulation of the receptor. It has been suggested that opioid receptor phosphorylation is the mechanism by which this desensitization and down- regulation occurs. When MOR1TAG was stably expressed in both neuroblastoma neuro2A and human embryonic kidney HEK293 cells, the opioid agonist [D- Ala2,MePhe4,Gly5-ol]enkephalin (DAMGO) induced a time- and concentration- dependent phosphorylation of the receptor, in both cell lines, that could be reversed by the antagonist naloxone. Protein kinase C can phosphorylate the receptor, but is not involved in DAMGO-induced MOR1TAG phosphorylation. The rapid rate of receptor phosphorylation, occurring within minutes, did not correlate with the rate of the loss of agonist-mediated inhibition of adenylyl cyclase, which occurs in hours. This lack of correlation between receptor phosphorylation and the loss of response was further demonstrated when receptor phosphorylation was increased by either calyculin A or overexpression of the G-protein receptor kinases. Calyculin A increased the magnitude of MOR1TAG phosphorylation without altering the DAMGO-induced loss of the adenylyl cyclase response. Similarly, when μ- and δ-opioid (DOR1TAG) receptors were expressed in the same system, overexpression of β-adrenergic receptor kinase 2 elevated agonist-induced phosphorylation for both receptors. However, in the same cell lines under the same conditions, overexpression of β-adrenergic receptor kinase 2 and β-arrestin 2 accelerated the rate of DPDPE- but not DAMGO-induced receptor desensitization. Thus, these data show that phosphorylation of MOR1TAG is not an obligatory event for the DAMGO-induced loss in the adenylyl cyclase regulation by the receptor.