Loss of VAMP2, VAMP3, and VAMP8 affects the kinetics and the extent of platelet secretion. [³H]-5-[HT] (serotonin)–labeled platelets from WT, V8^−/−, V2^Δ3^Δ, and V2^Δ3^Δ8^−/− mice were prepared as described in supplemental Methods. The release of [³H]-5-[HT] from dense granules (A,D), PF4 from α granules (B,E), and β-hexosaminidase from lysosomes (C,F) was measured, and percentage secretion was calculated as described in supplemental Methods. (A-C) For the thrombin dose-response experiment, platelets were stimulated for 2 minutes with the indicated concentrations of thrombin. (D-F) For the time-course experiments, platelets were stimulated with 0.05 U/mL thrombin for the indicated times Data are mean ± standard error of the mean of triplicate measurements and are representative of ≥3 independent experiments.