—A rapid accumulation of [3H]GABA occurs in slices of rat cerebral cortex incubated at 25° or 37° in a medium containing [3H]GABA. Tissue medium ratios of almost 100:1 are attained after a 60 min incubation at 25°. At the same temperature no labelled metabolites of GABA were found in the tissue or the medium. The process responsible for [3H]GABA uptake has many of the properties of an active transport mechanism: it is temperature sensitive, requires the presence of sodium ions in the external medium, is inhibited by dinitrophenol and ouabain, and shows saturation kinetics. The estimated Km value for GABA is 2·2 × 10−5m, and Vmax is 0·115 μmoles/min/g cortex. There is only negligible efflux of the accumulated [3H]GABA when cortical slices are exposed to a GABA-free medium. [3H]GABA uptake was not affected by the presence of large molar excesses of glycine, l-glutamic acid, l-aspartic acid, or β-aminobutyrate, but was inhibited in the presence of l-alanine, l-histidine, β-hydroxy-GABA and β-guanidinopropionate. It is suggested that the GABA uptake system may represent a possible mechanism for the inactivation of GABA or some related substance at inhibitory synapses in the cortex.