The most prevalent single-nucleotide polymorphism (SNP) A118G in the human µ-opioid receptor gene predicts an amino acid change from an asparagine residue to an aspartatic residue in amino acid position 40. This N40D mutation, which has been implicated in the development of opioid addiction, was previously reported to result in an increased β-endorphin binding affinity and a decreased potency of morphine-6-glucuronide. Therefore, in the present study we have investigated whether this mutation might affect the binding affinity, potency, and/or the agonist-induced desensitization, internalization and resensitization of the human µ-opioid receptor stably expressed in human embryonic kidney 293 cells. With the exception of a reduced expression level of N40D compared to human µ-opioid receptor (hMOR) in HEK293 cells, our analyses revealed no marked functional differences between N40D and wild-type receptor. Morphine, morphine-6-glucuronide and β-endorphin revealed similar binding affinities and potencies for both receptors. Both the N40D-variant receptor and hMOR exhibited robust receptor internalization in the presence of the opioid peptide [d-Ala2,N-MePhe4,Glyol5]enkephalin (DAMGO) and β-endorphin but not in response to morphine or morphine-6-glucuronide. After prolonged treatment with morphine, morphine-6-glucuronide or β-endorphin both receptors showed similiar desensitization time courses. In addition, the receptor resensitization rates were nearly identical for both receptor types.