GABA turnover can be measured by following the conversion rates of 13C glutamate into 13C GABA. This is not possible on cortex, hippocampus, and cerebellum presumably because these contain abundant glutamatergic nerves where 13C glutamate is not converted into GABA. Since it is impossible to correct for this discrepancy, the increment in GABA levels following the intraventricular injection of the GABA transaminase inhibitor, gabaculine and the decrement in GABA levels following subcutaneous injection of the glutamate decarboxylase inhibitor, isoniazid, have been used to estimate GABA turnover rates in various rat brain areas. In those structures where GABA turnover can be measured isotopically, the fractional rate constants measured by perturbing GABA steady state are one-tenth of those measured with tie isotopic method. The reasons for the discrepancy between the two methods are being investigated. Meanwhile, the isotopic method and the enzyme inhibitor methods can be used for comparative studies of drug action even though the absolute values obtained are discrepant.