The central effects of the 5-(o-fluorophenyl)-3-hydroxybenzodiazepines SAS 643, SAS 646 and Ro 7–5205 were investigated and compared with those of diazepam. It was found that, in rats, these drugs have a pharmacological profile very close to that of diazepam. They possessed a high antimetrazol activity, a muscle-relaxant effect and a depressive action upon exploratory behavior. Moreover, they facilitated behavior suppressed by punishment and caused a synchronization of the EEG pattern. With regard to the relative potencies of the compounds, SAS 646 was found to be the most potent drug by far; it was 4–10 times more potent than diazepam, while SAS 643 was equipotent to the latter. This suggests the usefulness of the new compounds for clinical use. The structure-activity relationships indicate that the general rules stated by Stembach et al are valid also for the group of the 5-(O-fluorophenyl)-3-hydroxybenzodiazepines.