1. Citalopram (Lu 10-171), a new bicyclic phthalane derivative, is an extremely potent inhibitor of neuronal serotonin (5-HT) uptake but has no effect on the uptake of noradrenaline (NA) and dopamine (DA). 2. Citalopram has no antagonistic activity towards DA, NA, 5-HT, histamine, gamma aminobutyric acid (GABA), acetylcholine, and morphine receptors. In this way it clearly deviates from many old and new antidepressant drugs which have antagonistic effects towards some of these transmitters. 3. In contrast to many tricyclic antidepressants citalopram is devoid of cardiotoxic effects, even when animals are exposed to concentrations far above the therapeutic level. 4. In man citalopram is metabolized to compounds which are also potent 5-HT-uptake inhibitors without effect of NA uptake and which are found in lower concentrations than citalopram itself. 5. In account of its extreme specificity as a 5-HT-uptake inhibitor citalopram should be considered as an experimental tool of the utmost importance. In preliminary clinical experiments citalopram has shown a clear antidepressant effect. This property together with the absence of troublesome anticholinergic adverse effects and cardiotoxic effects also make citalopram a most promising antidepressant drug.