Human platelets loaded with the fluorescent Ca2+ indicator quin2 and with different radioactive compounds including [3H]serotonin, [14C]arachidonic acid (AA) and [32P]orthophosphate were stimulated by thrombin under conditions producing secretion. In the absence of external Ca2+ (Ca2+e), cytoplasmic free [Ca2+], [Ca2+]i, increased to 340 nM, against 1685 nM at 1 mM [Ca2+]e. In both cases, diglyceride and phosphatidic acid production proceeded at the same rate, whereas AA release was inhibited at low [Ca2+]i. It is concluded that, at variance with phospholipase A2, phospholipase C activation does not depend on [Ca2+]i. These results give further support to the hypothesis of a Ca2+-independent pathway of cell activation involving phospholipase C and protein kinase C.