Quinonoid dihydropteridine reductase (QDPR) regenerates tetrahydrobiopterin (BH4), an indispensable cofactor for synthesizing catecholamines and serotonin. We evaluated the platelet aggregation of QDPR gene knockout (Qdpr ^-/-) mice. Citrated blood was collected from wild type (Qdpr ^+/+) mice and Qdpr ^-/- mice (OYC35, Lexicon Pharmaceuticals Inc.). Platelet rich plasma (PRP) was obtained by centrifuging the blood, and adjusted platelet count to 250,000/μL. Platelets were stimulated with collagen (3～4 μg/mL) or ADP (5～20 μM) and aggregation was measured by light transmission method for 10 minutes. Intraplatelet serotonin was quantified by high-performance liquid chromatography (HPLC) with electrochemical detection. The Qdpr ^-/- mice significantly decreased the area under the aggregation curve (AUC) stimulated with collagen (Qdpr ^+/+: 6,061±600, Qdpr ^-/-: 3,424±514, p<0.05 by Tukey-Kramer test) and ADP (Qdpr ^+/+: 4,197±295, Qdpr ^-/-: 2,452±266, p<0.05). The intraplatelet serotonin content was significantly decreased in the Qdpr ^-/- mice (Qdpr ^+/+: 48.1±4.0 pmol/10⁶ platelets, Qdpr ^-/-: 22.1±2.8 pmol/10⁶ platelets, p<0.01). These results indicate that the Qdpr ^-/- mice suppresses secondary platelet aggregation via serotonin.