Opioid receptors mediate multiple biological functions through their interaction with endogenous opioid peptides as well as opioid alkaloids including morphine and etorphine. Previously we have reported that the ability of distinct opioid agonists to differentially regulate mu-opioid receptor (mu OR) responsiveness is related to their ability to promote G protein-coupled receptor kinase (GRK)-dependent phosphorylation of the receptor (1). In the present study, we further examined the role of GRK and beta-arrestin in agonist-specific regulation of the delta-opioid receptor (delta OR). While both etorphine and morphine effectively activate the delta OR, only etorphine triggers robust delta OR phosphorylation followed by plasma membrane translocation of beta-arrestin and receptor internalization. In contrast, morphine is unable to either elicit delta OR phosphorylation or stimulate beta-arrestin translocation, correlating with its inability to cause delta OR internalization. Unlike for the mu OR, overexpression of GRK2 results in neither the enhancement of delta OR sequestration nor the rescue of delta OR-mediated beta-arrestin translocation. Therefore, our findings not only point to the existence of marked differences in the ability of different opioid agonists to promote delta OR phosphorylation by GRK and binding to beta-arrestin, but also demonstrate differences in the regulation of two opioid receptor subtypes. These observations may have important implications for our understanding of the distinct ability of various opioids in inducing opioid tolerance and addiction.