This study was aimed at assessing monoamine catabolites plasma levels in depressed patients and healthy volunteers. Plasma levels of 3-methoxy-4-hydroxyphenylglycol (MHPG), homovanillic acid (HVA), and 5-hydroxyindoleacetic acid (5-HIAA) of 21 control subjects and 26 depressed patients (according to DSM III-R criteria) were measured at baseline (day 0) and day 4, day 7, day 30 of prescribed antidepressant treatment. The clinical assessment, at baseline as well as during treatment, used the Hamilton depression rating scale and the BPRS. Our data show the interest of these results in predicting response. The respondent patients showed a significant decrease in plasma MHPG level at J7, contrary to non-respondent patients. Moreover, a positive correlation between plasma levels of MHPG and HVA before any prescribed antidepressants was found only with respondent patients. The lack of correlation for non-respondent patients can suggest that the relationships between this monoamine systems should be disrupted in these patients. Significant relationships appear between clinical symptoms and plasma catabolites, allowing us to consider new physiopathological aspects of the depressive picture. The 3 monoamines seemed involved in sleep disorders. Perturbations of norepinephrine and serotonin metabolism could intervene in suicidal ideation and behaviour. Motor activity was associated with a modification in dopamine and serotonin metabolism. Moreover significant correlations were observed between items referring to thought content and monoamine plasma catabolites such as MHPG and blunted affect, 5-HIAA and obsessions, HVA and guilt feelings, devalorization and without hope items.(ABSTRACT TRUNCATED AT 250 WORDS)