The inositol 1,4,5-trisphosphate (IP3) content and intracellular free Ca2+ ([Ca2+]i) level in response to kappa-opioid receptor stimulation with selective kappa-opioid receptor agonists, dynorphin-(1-13) and trans-3,4-dichloro-N-[2-(1-pyrrolidinyl)cyclohexyl]benzeacetamidel (U50,488H) were determined in ventricular myocytes. Both IP3 and [Ca2+]i were increased following kappa-opioid receptor stimulation. The responses of IP3 and [Ca2+]i to kappa-opioid receptor stimulation were abolished in myocytes of rats that had received chronic injection of U50,488H for 4 days. kappa-Opioid receptor stimulation with U50,488H also reduced the [Ca2+]i transient, induced by electrical stimulation and caffeine, both known to mobilize [Ca2+]i. The effect was abolished after the myocytes had been incubated with U50,488H at a subthreshold concentration for its effect on [Ca2+]i for 24 h. The present study showed for the first time that, upon the development of tolerance to a kappa-opioid receptor agonist, the responses of IP3 and [Ca2+]i to kappa-opioid receptor stimulation were abolished. The lack of response in [Ca2+]i was due to a failure of mobilization of Ca2+ from its intracellular pool. Further study is needed to determine the events that occur after the kappa-opioid receptor stimulation to production of IP3 upon the development of tolerance to a kappa-opioid.