Abstract The effect of γ-aminobutyric acid (GABA) on protein synthesis in vitro by slices and homogenates of guinea pig brain was studied. GABA was shown to significantly increase the incorporation of l-[1- 14C]leucine, alanine, and phenylalanine into protein by slices and homogenates of cortex and forebrain. The stimulatory effect of GABA upon protein synthesis was greater in slices than homogenates, and could not be demonstrated in cerebellar slices or homogenates. GABA stimulation of protein synthesis was demonstrable only when additional non-radioactive amino acids were present in the incubation medium. Tissue which had been exposed to GABA during incubation showed a greater percentage of heavy polysomes, as judge by ultra-violet absorption, than did tissue from control flasks. GABA did not change the incorporation of phenylalanine into polyphenylalanine in response to polyuridylic acid. l-Noradrenaline was shown to stimulate the incorporation of l-[1- 14C]leucine into protein, and this increase could be blocked by dichloroisoproterenol.