Tetrahydrobiopterin (BH_4), an endogenous active compound, plays a crucial role in the biosynthesis of aromatic monoamines and nitric oxide as well as the conversion of phenylalanine to tyrosine. BH_4 deficiency causes extensive dysfunction of the body systems including the nervous and cardiovascular systems. Supplementation of BH_4 has not prevailed despite its potential effectiveness against BH_4 deficiency. The level of BH_4 in the body is maintained by keeping a balance between its de novo synthesis and urinary excretion. The urinary excretion of BH_4 is attenuated by its reabsorption at the renal tubules under ordinary conditions. Orally administered BH_4 is absorbed very efficiently, and subsequently a large increase in blood BH_4 concentration occurs. However, the increase in blood BH_4 concentration provokes its active secretion across the renal epithelial cells as well as glomerular filtration, resulting in a decrease in the increased blood level of BH_4 to its ordinary level. In this review, we discuss both the roles of BH_4 as a coenzyme, and the intestinal absorption, tissue distribution and urinary excretion.