The neurobiological functions of ascorbate have both intra- and extracellular sites of action. Intracellularly, it participates predominantly in enzymic and transport reactions for neurotransmitter and hormone biosynthesis. Ascorbate is the cofactor for the dopamine beta-hydroxylase and peptidylglycine alpha-amidating monooxygenase systems, which catalyze the synthesis of norepinephrine and a variety of alpha-amidated peptides, respectively. The localization of these enzymes within the neurotransmitter- or hormone-containing storage vesicle requires a system for the constant regeneration of ascorbate to the reduced form. In fact, ascorbate participates in its own regeneration as a component of the vesicular electron-transport system. In addition to the roles of ascorbate in messenger synthesis, it is secreted from cells from different subcellular compartments. The extracellular role(s) of ascorbate are still unknown, although its interaction with and modification of plasma membrane proteins suggests some modulatory function.