Biopterin concentrations in red blood cells were higher in patients with chronic hyperphenylalaninaemia than following a single oral dose of phenylalanine, although the plasma concentrations were similar. These findings suggest that tetrahydrobiopterin is synthesized in immature red blood cells in response to hyperphenylalaninaemia. The less marked increase in plasma biopterin suggests slow release from a tissue reservoir. The maintenance of a differential between erythrocyte and plasma biopterin concentrations suggests that some mechanism retains biopterin intracellularly.