(1) Acoustic stimulation at 20/sec was performed on cats both in the waking. state and anaesthetized by pentobarbital or chloralose. (2) After 45 min of stimulation, samples of the auditory region of the cerebral cortex were examined at the electron microscope. Vesicular densities in axons and terminals of the 4th and 5th layers of control and stimulated cortices were evaluated and compared. Axons were grouped into categories of cross-sectional area, the areas rising in steps of 0.1 sq. μm. (3) In waking cats the vesicular density rose in axonal terminals of 0.1 and 0.3 sq. μm categories, whereas it decreased in terminals of 0.2 and 0.4–0.6 sq. μm areas. In non-synapsing axons only diminution of density was found. (4) In cats anaesthetized with pentobarbital, significant decreases of vesicular density were observed on stimulation, with the exception of 0.3 sq. μm axons, which showed no change. (5) In cats anaesthetized with chloralose there was a decrease in density only in the 0.1 and 0.3 sq. μm categories; in others no significant change was detected. (6) It is suggested that axonal terminals. which showed a significant decrease of vesicular density, represent cortical endings of the specific thalamocortical pathway, while those with elevated vesicle counts may belong to the reticulocortical neurones.