In the rat mammary tumoral cell line (WRK1 cells), vasopressin was previously described to stimulate a phospholipase C. In this study, we have analysed the effect of vasopressin both on intracellular calcium mobilization and on the accumulation of inositol phosphates. Maximal concentration of vasopressin simultaneously induces an accumulation of Ins(1,4,5)P3 and a rise of intracellular calcium concentration. Both these two phenomena are transient and exhibit similar kinetics. A sustained accumulation of InsP2, Ins(1,3,4)P3 and InsP are observed later. Yet no stimulation of InsP4 can be objectified. These results indicate that Ins(1,4,5)P3 is the major inositol phosphate involved in intracellular calcium mobilization.