Abstract— Slices of rat cerebral cortex were labelled by incubation with [3H]γ-aminobutyric acid (GABA) and homogenized in isotonic sucrose. The subcellular distributions of endogenous GAB A, [3H]GABA and glutamate decarboxylase (GAD) were studied by density gradient centrifugation. The subcellular distributions of the labelled and endogenous amino acid were remarkably similar, indicating that [3H]GABA is taken up into the endogenous GABA pool. About 40 per cent of both endogenous and [3H]GABA were recovered in particles which were tentatively identified as synaptosomes from their equilibrium density and sensitivity to osmotic shock. In slices labelled with [3H]GABA and [14C]α-aminoisobutyric (AIB) acid, significantly more [3H]GABA was recovered in paniculate fractions than [14C]AIB. About 80 per cent of the enzyme GAD was also recovered in the same particle fractions which contained [3H]GABA and endogenous GABA. Evidence is presented which suggests that a loss of particle-bound GABA occurs during subcellular fractionation procedures.