The accumulation rates of 3,4'-dihydroxyphenylalanine (DOPA) and 5-hydroxytryptophan (5-HTP) after inhibition of aromatic amino acid decarboxylase (AADC) by 3-hydroxybenzylhydrazine (NSD 1015) or 1-(DL-seryl)-2- (2,3,4-trihydroxybenzyl)hydrazine (Ro 4-4602) have widely been used as measurements of the in vivo synthesis rates of monoamines. However, the values of dopamine (DA) turnover in rat striatum obtained using these drugs are much lower than values obtained by other methods. This discrepancy prompted us to further investigate the AADC inhibitor 1-(3-hydroxybenzyl)-1-methylhydrazine (NSD 1034) which earlier has been shown to give a DOPA accumulation rate in the striatum of the same magnitude as other measures of DA turnover. NSD 1034 was found to give a more than twofold higher DOPA accumulation rate than NSD 1015, NSD 1024, NSD 1039, NSD 1055 and Ro 4-4602 in the striatum. Also, in the limbic region and the hemispheres, but not in the substantia nigra, the DOPA accumulation was higher after NSD 1034 than after NSD 1015, but the difference was less pronounced. There was, however, no difference in 5-HTP accumulation between the drugs in any of the brain parts investigated. Although the DOPA accumulation rates are higher after NSD 1034 than after NSD 1015, the NSD 1015-induced DOPA accumulation seems to be more sensitive to changes in dopamine receptor occupancy. The different DOPA accumulation rates obtained with NSD 1015 and NSD 1034 are not due to differences in MAO inhibition, to interference with classical DA receptors, or to different degrees of AADC inhibition, but to an ability of NSD 1034 to stimulate DA synthesis. In addition, under certain conditions NSD 1034 also has a DA releasing action, like amphetamine. It is proposed that NSD 1034 and amphetamine stimulate DA synthesis and release by a common mechanism. The low value of DA synthesis rate, obtained when measured as DOPA accumulation after NSD 1015, is due to a substantial efflux of DOPA from the brain. The efflux of DOPA is equally large after NSD 1034 but the loss is compensated for by an increase in DOPA synthesis.