Changes in gamma-aminobutyric acid (GABA) occurring in the presence and in the absence of GABA-containing nerve terminals were estimated in rats in which the dense GABA projection to the substantia nigra was surgically destroyed on one side of the brain. The net increase in GABA of the denervated nigra was compared with that of the intact nigra at various times after a single injection of gama-vinyl-GABA, which irreversibly inhibits GABA transaminase. Total GABA reached a maximum within 12 hours, but the GABA pool associated with nerve terminals did not increase until 36 hours and peaked at 60 hours. The onset and peak of anticonvulsant activity against maximal electroshock seizures directly paralleled the time course for the increase in GABA in nerve terminals, but was not positively correlated with that independent of the terminals. This result supports the concept that elevating GABA in nerve terminals facilitates GABA-mediated synaptic transmission and predicts anticonvulsant activity.