Phosphate activated glutaminase (PAG) was assayed in whole homogenate and synaptosomes of cerebral cortex from normal or fasted for 120 h rats. The specific activity (s.a.) of PAG was found diminished by 25% in the whole homogenate from the fasted animals compared to the normal. On the contrary, fasting did not affect PAG s.a. of the synaptosomal fraction. Reconstitution experiments showed that when the deproteinized supernatant of the 12,500 g centrifugation from the fasted rats was added to the synaptosomes from either fed or fasted animals the PAG activity was diminished but there was no change when the corresponding supernatant from the fed animals was added to the synaptosomes from both conditions. When leucine at 5mM was added to the homogenate or to synaptosomes from fed or fasted animals the s.a. of PAG was significantly decreased. Even in the presence of aminooxyacetate the effect of leucine was observed. Branched chain amino acids i.e. leucine, isoleucine and valine at 0.5 mM each added to synaptosomes again decreased PAG activity. The addition of ketone bodies had no effect. It is suggested that leucine, because PAG has been implicated in the supply of transmitter glutamate, might be an important regulator of the pool of this neurotransmitter.