An accumulation of L-dihydroxyphenylalanine (DOPA) in the median eminence of female rats treated with 3-hydroxybenzylhydrazine (NSD 1015), and inhibitor of aromatic L-amino acid decarboxylase (DOPA decarboxylase) activity, was associated with a decreased concentration of dopamine in the median eminence and pronounced reduction in the release of dopamine into hypophysial portal blood. The amount of dopamine released during 1 h into hypophysial portal blood of vehicle-treated rats represented 18% of the amount of DOPA that accumulated in 1 h in the median eminence of rats treated with NSD 1015. The reduction in the concentration of dopamine in the plasma of blood from a single hypophysial portal vessel after the inhibition of DOPA decarboxylase activity was associated with a concomitant increase in the concentration of PRL in the plasma of arterial blood. In contrast to that of dopamine, the concentration of norepinephrine as well as the concentration of epinephrine in hypophysial portal plasma was the same or slightly greater in animals treated with NSD 1015 than in animals treated with the solvent vehicle. These findings are supportive of the view that the release of dopamine from the tuberoinfundibular neurons is highly dependent on the rate of conversion of DOPA to dopamine and that the rate of synthesis of DOPA is a major factor in the control of the rate of release of dopamine into hypophysial portal blood.