The selective inhibitors of neuronal 5-hydroxytryptamine (5-HT) and noradrenaline (NA) uptake, zimelidine and desipramine, were compared in a double blind crossover study of 40 inpatients with endogenous depression. The clinical effects of these two drugs and some biochemical variables related to the monoamine systems were studied during 4 weeks' treatment. Patients with a low pretreatment level of 5-hydroxyindoleacetic acid (5-HIAA) in the cerebrospinal fluid (CSF) (less than 20 ng/ml) responded significantly better to zimelidine treatment than those with a high pretreatment level (greater than 20 ng/ml). In the group treated with desipramine no difference in therapeutic outcome was obtained between patients with low and high pretreatments levels of 5-HIAA in the CSF. Attempts to correlate the steady state plasma concentrations of zimelidine, norzimelidine and desipramine with the therapeutic effect were unsuccessful. The plasma concentration of norzimelidine demonstrated a significant (P less than 0.05) positive correlation with age. The mean value of the uptake of 14C-5-HT in the patient's platelets, when measured before the treatment, was significantly (P less than 0.05) lower than found in a control group. Zimelidine, mainly via its metabolite norzimelidine, caused a pronounced inhibition of uptake of 14C-5-HT in platelets, decrease in whole blood 5-HT and inhibition of accumulation of 14C-5-HT in rat hypothalamic synaptosomes incubated in the patients plasma. Desipramine produced a slight inhibition of accumulation of 14C-5-HT in rat synaptosomes, but a marked inhibition of uptake of 14C-5-HT in the patient's platelets and a decrease in whole blood 5-HT. The accumulation of 3H-NA in rat synaptosomes incubated in the patient's plasma was strongly inhibited by desipramine treatment but only slightly affected by zimelidine.