The subject of this study was the disappearance of 3, 4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) from the brain of rats treated with pargyline and pargyline in combination with tropolone. The DOPAC decline was exponential after pargyline treatment, while the HVA decline became a first order elimination when in addition to monoamine oxidase, catechol-O-methyl transferase was also inhibited. The turnover of the metabolites was calculated by multiplying the steady state level by the fractional rate constant (k). The DOPAC turnover was found to be 23.3 nmol/g/h in the corpus striatum and 22.6 nmol/g/h in the mesolimbic structures. The HVA turnover was 11.2 nmol/g/h in the corpus striatum and 6.7 nmol/g/h in the mesolimbic structures. The data showed that under control conditions DOPAC is only partially O-methylated to HVA, while the formation of HVA via the methoxytyramine pathway is unlikely. DOPAC turnover therefore probably approximates dopamine turnover.