Tetrahydrobiopterin (BH4) is an essential cofactor for tryptophan hydroxylases, which is limiting step in serotonin (5-hydroxytryptamine, 5-HT) synthesis. We have previously reported the quinonoid dihydropteridine reductase (QDPR), BH4 regeneration enzymes, gene knockout (Qdpr^-/-) mice suppress platelet aggregation (92th congress). We further investigate its mechanism by 5-HT recovery using 5-hydroxytryptophan (5-HTP) supplement, precursor of 5-HT. Wild type (Qdpr^+/+) and Qdpr^-/- mice (OYC35, Lexicon Pharmaceuticals Inc.) were injected subcutaneously with 10 mg/kg of 5-HTP twice a day for 3 days. Intraplatelet 5-HT was quantified by high-performance liquid chromatography (HPLC) with electrochemical detection. Platelet aggregation were measured using platelet rich plasma (PRP), adjusted platelet count to 270,000/μL. The change in light transmission correspond to aggregation reaction was monitored after stimulation with either ADP or collagen. The intraplatelet 5-HT of Qdpr^-/- mice with 5-HTP supplement was recovered from 21.9±3.1 pmol/10⁶ platelets to 75.7±4.0, similar level of Qdpr^+/+ mice as 61.3±5.7. The area under the aggregation curve (AUC) of ADP-induced response was also recovered from 2,275±157 to 4,831±451 by 5-HTP administration correspond level to 5,436±696 of Qdpr^+/+ mice. The AUC of collagen-induced response revealed similar effects (Qdpr^+/+: 6,493±368, Qdpr^-/-: 4,402±589, Qdpr^-/-+5-HTP: 5,682±403). These results indicate that internal 5-HT disturbance is responsible for reduction of platelet aggregation in Qdpr^-/- mice.