Interactions between selective opioid agonists acting at m- and d-opioid receptors were evaluated by co-administering a low-effective dose of the selective m-opioid receptor agonist ohmefentanyl (OMF) with sequentially increasing doses of the selective d-opioid receptor agonist [D-Pen2, D-Pen5] enkephalin (DPDPE). Microphysiometer was used to measure the extracellular acidification rate (ECAR) of living cells in real-time, which re-ected the functional activity after agonistreceptor binding. The synergy (i.e. a more than additive effect) was observed with combinations of these two opioid agonists on differentiated SH-SY5Y cells functionally expressing both m- and d-opioid receptors. The demonstration of the synergy suggests that the agonists of the subtypes of opioid receptors can interact at cellular level.