Interaction of the µ-opioid receptor (MOP) with selected ligands was investigated in live cells using advanced imaging by confocal laser scanning microscopy integrated with fluorescence correlation spectroscopy and fluorescence cross-correlation spectroscopy. In PC12 cells stably transformed to express the fluorescently labeled MOP-enhanced green fluorescent protein construct, two pools of MOP were identified that could be discriminated by differences in their lateral mobility in the cell membrane. The majority of MOP receptors (80±10%) were characterized by a diffusion coefficient D_{MOP, 1} = (4±2) x 10^-11 m² s^-1, compared with the slowly moving fraction, D_{MOP, 2} = (4±2) x 10^-12 m² s^-1. On stimulation with selected agonists ([D-Ala²,N-MePhe⁴,Gly-ol⁵]enkephalin, enkephalin-heptapeptide Tyr-Gly-Gly-Phe-Met-Arg-Phe, morphine, and methadone), surface density of the MOP decreased, whereas the lateral mobility increased. In contrast, antagonists (naloxone and naltrexone) "froze" the receptor in the membrane, i.e., increased MOP surface density and decreased lateral mobility. Agonist activation was also accompanied by pronounced changes in the dynamics of plasma membrane lipids, as revealed by the general lipid marker 1,1`-dioctadecyl-3,3,3`,3`-tetramethylindocarbocyanine perchlorate dye. The results provide new information about MOP activation in live cells at the molecular level, with a special focus on the dynamics of the intricate interplay between this receptor and the surrounding lipids.—Vukojevi, V., Ming, Y., D'Addario, C., Hansen, M., Langel, Ü., Schulz, R., Johansson, B., Rigler, R., Terenius, L. µ-Opioid receptor activation in live cells.