1 The rate of synthesis of gamma-aminobutyric acid (GABA) in the cortex, hippocampus and striatum of rat brain was assessed by measuring the linear rate of accumulation of GABA following injection of amino-oxyacetic acid (AOAA). 2 Five min after a single electrically induced seizure there was a rise in GABA content in these brain regions and an almost total inhibition of the rate of synthesis. 3 Five min after seizure induced by the inhalant convulsant flurothyl there was no rise in GABA content in these brain regions but a similar marked degree of inhibition of GABA synthesis. 4 Two hours after the convulsion the rate of GABA synthesis had returned to control values in all three brain regions. 5 A single convulsion did not alter the glutamic acid decarboxylase activity in these brain regions either in the absence or presence of added co-factor (pyridoxal phosphate). 6 Evidence for an inhibition of GABA release following a convulsion which may be associated with the inhibition of GABA synthesis is presented in the following paper.