The role of brain catecholamine (CA) activity in the neuroendocrine regulation of the GnRH-LH system in polycystic ovary syndrome (PCO) was investigated by high-performance liquid chromatography (HPLC) with electrochemical detector. We measured urinary dopamine (DA), noradrenaline (NA), adrenaline (A), vanillylmandelic acid (VMA), homovanillic acid (HVA), 3,4-dihydroxyphenylacetic acid (DOPAC) and total 3-methoxy-4-hydroxyphenylglycol (MHPG) levels in a group of 12 women with PCO before and during peripheral dopa-decarboxylase blockade, by carbidopa. HVA and DOPAC concentrations were significantly lower (P less than 0.001 and P less than 0.005, respectively) in PCO patients compared with twelve control subjects in early follicular phase, whereas total MHPG concentrations and MHPG/VMA ratio were significantly higher (P less than 0.005) in PCO patients. Moreover, HVA and DOPAC concentrations in PCO patients were similar to those of the control subjects in preovulatory phase, while MHPG concentrations remained higher in PCO patients (P less than 0.01). DA, NA, A and VMA concentrations were similar to those of control subjects in both phases of the cycle. During carbidopa administration the concentrations of all urinary CAs and metabolites were unchanged, except those of DA which dropped markedly (P less than 0.001). These data suggest that (1) an altered central catecholamine metabolism consisting of DA deficiency and NA excess is present in PCO, and (2) the site of DA deficiency may be located in the hypothalamus.