Part of the biochemical research in the area of lithium (Li) therapies of mood disorders was focused on interference with signaling initiated by GPCR. Although opioids were traditionally associated with pain and opioid dependence, an increasing evidence links the opioid receptors (OR) and OR-mediated activation of G proteins to the psychiatric disorders. The functional state of δ-OR signaling in live cells exposed to a therapeutic concentration of Li for a prolonged period of time (weeks) was tested in this work together with a biophysical state of plasma membrane (PM). HEK293 cells stably expressing δ-OR were cultivated in the presence or absence of 1 mM LiCl for 7 (L7 Li-treated, C7 control cells) or 21 days (L21 and C21) and three independent PM preparations were isolated from each of four types of cells. Level of δ-OR in PM was determined by specific radioligand [3H]DADLE binding and immunoblot assays; the functional coupling between δ-OR and G proteins was determined as DADLE-stimulated high-affinity [35S]GTPγS binding. Li interaction with PM was characterized by fluorescent probes DPH, TMA-DPH and Laurdan. Values are means±SEM, compared by unpaired Student’s t-test (triplicate for each preparation). Li-treated cells exhibited the decreased amount of δ-OR. This was evidenced by both [3H]DADLE binding (L7 83±3%, p<0.05; L21 83±3%, p<0.05; appropriate controls 100%) and immunoblot assays (L7 69±6%, p<0.01; L21 60±3%, p<0.01). Relatively high stimulation of the basal level of [35S]GTPγS binding by 10-4M DADLE in C7 (p<0.05) was diminished in L7 (p>0.05). The same result was found when C21 (p<0.05) were compared with L21 (p>0.05). The steady-state anisotropy of DPH and TMA-DPH fluorescence determined in PM isolated from Li-treated cells was not significantly different when compared with controls (p>0.05). FLIM studies in live cells indicated that Li treatment did not significantly alter the lifetime of TMA-DPH fluorescence (p>0.05). Analysis of the biophysical state of PM by Laurdan generalized polarization (GPex) indicated significantly higher values of this parameter in PM prepared from Li cells when compared with controls. The average value of GPex at 25°C in PM prepared from L7 cells (0.216±0.001, p<0.01) was significantly higher than in C7 (0.185±0.001). The same result was obtained in PM prepared from L21 (0.175±0.001, p<0.01) and C21 (0.145±0.002) and confirmed at 40°C. We conclude that prolonged exposure of δ-OR-HEK293 cells to 1 mM Li results in down-regulation of δ-OR level and attenuation of δ-OR-G protein coupling. This change proceeds in parallel with alteration of the polar head-group region of PM; the hydrophobic core of membrane is unchanged.