The role of Ca2+ in the stimulation of prostaglandin (PG) biosynthesis by vasopressin was investigated in rabbit renal-medullary interstitial cells in tissue culture. A decrease in extracellular Ca2+ to less than 25 microM did not affect basal PGE2 production, but inhibited PGE2 synthesis stimulated by vasopressin, angiotensin and the Ca2+ ionophore A23187 by 55, 65 and 95% respectively. The study of vasopressin-stimulated PGE2 synthesis in the absence of extracellular Ca2+ demonstrated that: (a) hormone-sensitive phospholipase activity was inhibited as measured by [3H]arachidonic acid release; (b) the maximal rate of vasopressin-stimulated activity was decreased without a change in the vasopressin concentration that evoked half-maximal stimulation of PGE2 synthesis; and (c) the Ca2+-channel blocker verapamil and the Ca2+-calmodulin antagonist trifluoperazine mimicked the inhibitory effects of removing extracellular Ca2+. These agents had no effect in the absence of Ca2+. In contrast with their effects on vasopressin action, neither the removal of extracellular Ca2+ nor the addition of verapamil altered the ability of hyperosmotic mannitol to increase PGE2 synthesis. These data are consistent with the hypothesis that a component of vasopressin-stimulated PGE2 biosynthesis involves the influx of extracellular Ca2+, followed by the activation of Ca2+-calmodulin-stimulated phospholipase(s).