Labelled amino acids, 14C-leucine, 3H-glycine, 14C-GABA, 14C-glutamic acid and 14C-aspartic acid were applied to the surface of the cat's auditory cortex in dried filter paper strips for 40 min. The distribution of the amino acids taken up by the cortex was examined by autoradiography.The right acoustic area was stimulated by 2 cps acoustic clicks, through an ear-phone. Leucine showed cellular localization which was considerably dispersed by cortical excitation. Glycine was concentrated in nerve cells, mainly in layers I-II, but excitation intensified and extended its incorporation. GABA was accumulated rather diffusely in layers I-II with scattered cellular labelling which was depressed by stimulation. Glutamic and aspartic acids did not show any characteristic distribution pattern. The authors emphasize that the incorporation of glycine seems to be a good indicator of neurons being in excitation. The localizations of GABA uptake shows close correlation with the site of its physiological action.