1. Chopped tissue from guinea-pig cerebral cortex carried out an energy-dependent incorporation of [14C]valine into protein. 2. At all times studied the nuclear fraction of the homogenized tissue accounted for about 25% of the total labelled protein. 3. Electrical stimulation at first increased, but subsequently decreased, the rate of incorporation of [14C]valine into protein of the chopped tissue. 4. The initial increase in the incorporation of [14C]valine into protein occurred in the nuclear fraction. At later times electrical stimulation decreased the incorporation into all the subcellular fractions, but the relative contribution of the nuclear fraction to the total labelled protein increased. 5. These changes are discussed in relation to the changes in the rates of respiration, glycolysis, high-energy phosphate content and intracellular Na+ and K+ concentrations, which were measured under the same experimental conditions as those used to study protein synthesis.
