Abstract Vasopressin increased intracellular free calcium concentration [Ca 2+] i in quin-2-loaded quiescent Swiss 3T3 cells. This effect of vasopressin was rapidly inhibited by biologically active tumour promoters including phorbol dibutyrate (PBt 2) and by the synthetic diacylglycerol 1-oleoyl-2-acetyl-glycerol (OAG). Prolonged pretreatment of Swiss 3T3 cells with PBt 2 causes a loss of protein kinase C activity (Rodriguez-Pena & Rozengurt, Biochem biophys res commun 120 (1984) 1053) . This pretreatment abolished the inhibition by PBt 2 or OAG of vasopressin-mediated increases in Ca 2+] i . Vasopressin also stimulated 45Ca 2+ efflux from cells pre-loaded with the isotope. This effect of the hormone was also inhibited by PBt 2. Prolonged pretreatment with PBt 2 prevented the inhibition of vasopressin-stimulated 45Ca 2+ release by PBt 2. Thus, protein kinase C stimulation inhibits vasopressin-mediated increases in [Ca 2+] i and 45Ca 2+ efflux apparently by blocking the increased release of Ca 2+ from an intracellular store caused by the hormone. These findings suggest that activation of protein kinase C may act as a feedback inhibitor to modulate ligand-mediated increases in [Ca 2+] i .