The effects of prolonged treatment of rats with zimelidine, 5, 12.5, and 25 ~tmol/kg PO twice daily for 2 weeks, on the accumulation of 14C-5-hydroxytryptamine (I#C-5-HT) and 3H-noradrenaline (3H-NA) in hypothalamic slices were studied. The concentrations of 5-HT and 5-hydroxyindoleacetic acid (5-HIAA) in whole brain, the concentration of 5-HT in whole blood and the in vitro labelling of receptors in cerebral cortex with 3H-5-HT, 3Hdihydroalprenolol (3H-DHA) and striatum with 3Hspiroperidol were also determined, and the concentrations of zimelidine and its demethylated metabolite norzimelidine in plasma and hypothalamus were analysed. The degree of inhibition of the accumulation of *4C-5-HT and ~H-NA was not changed or only slightly increased by prolonged treatment as compared to acute treatment with zimelidine, i.e. the 5-HT accumulation was more inhibited than the NA accumulation. The inhibition of ~C-5-HT accumulation was significantly correlated to the plasma and hypothalamic concentration of norzimelidine 14h after the last repeated administration. The 5-HT concentration in whole blood was markedly reduced at the same doses which produced inhibition of 5-HT uptake in brain, which indicates that the inhibition of 5-HT uptake in platelets and in neurons are similarly affected. The concentration of 5-HIAA in whole brain was reduced by both single and repeated administration of zimelidine, whereas the concentration of 5-HT was decreased only after prolonged treatment. The density of/~-adrenoceptors (binding of 3H-DHA) was significantly reduced by zimelidine, whereas 5-HT receptor binding (3H-5-HT) and dopamine receptor binding (3H-spiroperidol) were unchanged. It is concluded that the effects on 5-HIAA and 5-HT levels in brain and on the/?-adrenoceptors in cerebral cortex reflect pre-and post-synaptic regulation resulting from the uptake inhibition.