The mu opioid agonist DAMGO increases intracellular Ca2+ concentration and inositol triphosphate formation in Chinese hamster ovary cells stably expressing the mu opioid receptors rMOR1 and rMOR1B, a recently isolated isoform of rMOR1. The increase in intracellular Ca2+ concentration is due solely to mobilization from internal stores. Both the DAMGO-induced stimulation of inositol triphosphate and increase in intracellular Ca2+ concentration is similar for both variants with an EC50 of about 1 microM, which is about an order of magnitude less efficient than the inhibition of adenylyl cyclase (EC50 = 150 microM).