Cytoplasmic free [Ca 2 + ], [Ca 2 + ] i , was measured in human platelets using the intracellularly-trapped, fluorescent indicator quin2. Basal [Ca 2 + ] i with the Ca 2 + -ionophore ionomycin revealed apparent thresholds for shape-change, 5-HT release and aggregation of approx. 0.5μM, 0.8μM and 2 μM. Thrombin raised [Ca 2 + ] i to 3 μM fast enough for the Ca 2 + to have triggered the cell activation. However, thrombin released more 5-HT than ionomycin could, and in Ca 2 + -free medium thrombin evoked shape-change and secretion even when [Ca 2 + ] i remained near basal levels throughout, suggesting the existence of alternative triggers for shape-change and secretory exocytosis.