The results obtained by positron-emission tomography in an "in vivo" study on the baboon using a benzodiazepine (flunitrazepam) labeled with carbon-11 are presented. The specificity of "in vivo" binding of Flunitrazepam-11C was demonstrated by competition with Lorazepam in the brain, but it was not possible to verify the criterium of stereospecificity "in vivo". The preliminary results of a study carried out under the same conditions on RO 15 1788 11C show the interest of using this labeled antagonist as an "in vivo" ligand for the specific binding sites of benzodiazepines in positron-emission tomography.