HPLC coupled with electrochemical detection was used to make concurrent measurements of the rate of accumulation of 5-hydroxytryptophan and 3,4-dihydroxyphenylalanine in selected brain regions (striatum, nucleus accumbens, septum, medial periventricular hypothalamus) and thoracic spinal cords of rats treated with NSD 1015, an inhibitor of aromatic-L-amino-acid decarboxylase.