The modulating effect of serotonergic drugs on the striatal dopamine neurotransmission has remained controversial, and there are no published data on serotonin-dopamine interaction obtained from living human brain. Citalopram is a selective serotonin reuptake inhibitor widely used in the treatment of depression (20-40 mg/day). We measured the effects of acute (20 mg, per os) and chronic (20 mg/day for 14 days) doses of citalopram and placebo intake on [11C]-raclopride binding to striatal D2-receptors in eight healthy volunteers by using positron emission tomography. Although the effect magnitude was not large, the results indicate that chronic citalopram intake slightly decreases the raclopride binding which may reflect increased dopamine release in the striatum. In addition, after 14 days there was a high correlation between the citalopam plasma levels and the decrease in the [11C]-raclopride binding in both the caudate and the putamen, although statistically significant effect in the raclopride binding potential was more pronounced in the putamen. This report suggests functional interaction of brain dopaminergic and serotonergic systems in vivo in man.