Abstract— The level of saturation of glutamate decarboxylase (GAD) by cofactor, pyridoxal-5′-phosphate (pyridoxal-P), determined in synaptosomes prepared from substantia nigra tissue, was reduced from 45 to 28%; when ATP was included in the homogenizing medium to prevent nonspecific activation of GAD by endogenous pyridoxal-P. When the synaptosomes were incubated for 5–20 min at 37°C in Krebs-Ringer phosphate buffer (KRP), the level of saturation of GAD by cofactor decreased further, from 28 to 20%. Depolarization of the nigral synaptosomes by either high K+ (55 mM) or veratridine resulted in a significant increase in the level of GAD saturation by cofactor, to 32 and 41%. respectively. Omitting Ca2+ from the incubation medium blocked the depolarization-induced rise in the level of saturation. Depolarization with high K+ and veratridine also caused a significant decrease in the ATP concentration in the synaptosomes. No difference in ATP concentration was observed when the samples were incubated at 37°C for 5–20min or incubated in the absence of added Ca2+ with high K+. Results provide further evidence that in vivo brain GAD is largely unsaturated by cofactor and support the possibility that increased release and utilization of GABA may be associated with increases in the amount of pyridoxal-P endogenously bound to GAD in nerve terminals.