The influence of decreased endogenous gamma-aminobutyric acid (GABA) concentration on benzodiazepine receptor function was studied in the brain of living baboons. Positron emission tomography and the radiotracer [11C]flumazenil combined with electroencephalography were used to determine the pharmacological properties of two bezodiazepine receptors agonists, diazepam and bretazenil, in baboons pre-treated or not with DL-allylglycine (an inhibitor of GABA synthesis). Our results show that, in vivo, DL-allylglycine reduces the affinity of benzodiazepine receptors for their agonists without altering the intrinsic capability of agonists to allosterically modulate GABAergic transmission.