The energetics of active transport of gamma-aminobutyric acid (GABA) by neuronal synapses has been studied using preparations of synaptosomes from rat brain. The maximal (steady state) accumulation of GABA by synaptosomes was measured as a function of the transmembrane potassium concentration gradient (which is equal to the transmembrane electrical potential in this system), the sodium concentration gradient, and the external chloride concentration. The steady state [GABA]i/[GABA]o increased as the second power of [K+]i/[K+]o which indicates that GABA is transported with a net charge of +2. The GABA gradient was dependent on [Na+]o/[Na+]i to a power of between 1.2 and 2.1 but was almost independent of [Cl-]. It is concluded that a neutral GABA molecule is cotransported with 2 Na+ to give a net charge of +2. The coupling of GABA transport to the transmembrane electrical potential and the Na+ concentration gradient is sufficient to provide the energy for the measured GABA concentration gradient in rat brain synaptosomes.