We have examined the interaction between FK 506 and isoproterenol in their modulation of glutamate release from cerebrocortical nerve terminals (synaptosomes). Application of FK 506, an inhibitor of protein phosphatase 2B (calcineurin), resulted in a concentration-dependent potentiation of 4AP-evoked glutamate release. The beta-adrenergic receptor agonist isoproterenol and the membrane-permeable activator of protein kinase A Sp-cAMP also caused a significant increase in evoked glutamate release, which was occluded by FK 506 pretreatment. By studying the voltage-dependent Ca2+ influx with fura-2, we show that, while FK 506 and isoproterenol alone produced a potentiation of the 4AP-evoked increase in intracellular Ca2+, the addition of FK 506 abolished the isoproterenol-mediated potentiation of Ca2+ influx. Based on these results, we suggest that the interaction between the two substances in their potentiating effect occurs, at least in part, at the level of the voltage-dependent Ca2+ entry that affects cell excitability and glutamate release.