Fluorescence ratio imaging indicates that immobilized, aspirin-treated platelets, loaded with Fura-2, respond to inositol 1,4,5-trisphosphate- (InsP3)-generating agonists such as thrombin by high-frequency, irregular rises in cytosolic [Ca 2+ ] i with spikes that vary in peak level and peak-to-peak interval. This differs from the regular [Ca 2+ ] i oscillations observed inother, larger cells.