Fluoxetine (Lilly 110140) is a potent, specific serotonin (5-HT) uptake blocker which is being tested in man for antidepressant activity. One of 9 depressed patients receiving this drug developed a dystonic reaction, parkinsonian rigidity, and increased serum prolactin levels, all signs of decreased dopaminergic activity. Homovanillic acid levels also decreased in the cerebrospinal fluid of this subject. We postulate that fluoxetine, via the increase in 5-HT activity resulting from 5-HT uptake blockade, inhibited both the nigro-striatal and tubero-infundibular dopaminergic neurons. These results provide additional evidence for a linkage between serotonergic and dopaminergic neurons in man.