Tianeptine is a novel tricyclic antidepressant which, in marked contrast to other antidepressants, increases the uptake of 5-HT in brain tissue and blood platelet preparations ex vivo. In the present study it was shown, by microdialysis, that tianeptine (10 mg/kg i.p.), whether given as a single dose or as daily doses for 14 days, attenuated the K(+)-evoked rise of extracellular 5-HT in the hippocampus in vivo. Tianeptine (10 mg/kg i.p.), given acutely, did not reduce basal levels of 5-HT in hippocampal dialysates in the presence of the 5-HT reuptake inhibitor, citalopram. These results suggest mutually opposing effects of tianeptine and citalopram on neuronal uptake of 5-HT. Their significance in relation to the role of 5-HT in the action of antidepressants is discussed.