The source of urinary catecholamines and the mechanisms by which tyrosine administration raises these compounds were investigated in rats. Adrenalectomy blocked the tyrosine-induced rise in urinary epinephrine but not dopamine or norepinephrine. Following chemical sympathectomy with 6-hydroxydopamine (6-OHDA), the tyrosine-induced increase in urinary norepinephrine was diminished, but epinephrine and dopamine responses were unaffected. Cardiac norepinephrine, which is normally unchanged following tyrosine administration, became significantly elevated in 6-OHDA-pretreated animals. At the doses used in this study, tyrosine had no effect on the uptake or metabolic clearance of circulating catecholamines. We conclude that tyrosine augments the synthesis of the three catecholamines in sympathoadrenal cells by increasing the extent to which tyrosine hydroxylase is saturated with its amino acid substrate. Moreover, the effects of tyrosine on peripheral catecholamine synthesis may be enhanced under conditions of increased sympathetic activity.