The hypothesis that brain damage in phenylketonuria is related to inhibition of pyruvate kinase by phenylalanine was examined in rat brain in vivo. One hour after a single injection of phenylalanine into the rat, the brains were removed and completely frozen in less than a second. The concentration of phenylalanine in the brain was comparable to that found in phenylketonuric patients. Changes in brain glycolytic intermediates were consistent with inhibition of pyruvate kinase in vivo. The inhibition of pyruvate kinase was apparently compensated for by an increase in phosphoenolpyruvate; no decrease in adenosine triphosphate or creatine phosphate was found.