In vivo 19F nuclear magnetic resonance (19F-NMR) spectroscopy measurements of trifluorinated neuroleptics (fluphenazine--FL and trifluoperazine--TFP) were made in rat brain as well as in human brain. Animal studies were performed at 4.7 Tesla. Using rats that have been treated with FL over a period of 3 weeks, a NMR signal could be detected within 8-15 min. Following a single intravenous injection of TFP, brain levels could be monitored with a time resolution of 30 min. The first 19F-NMR examination of a patient was made at 3.0 Tesla 1 day after injection of FL decanoate (37.5 mg) in the course of which a signal could be detected within 30 min. It is expected that 19F-NMR will become an important tool in psychopharmacological research.