The changes which occur in the Berger rhythm in response to auditory stimulation under certain conditions of "mental set" were studied. The subject was presented a random series of high and low tones (250 and 120 d.v. respectively, and equated for loudness) under conditions of discriminatory response to high tone only and speed response to both tones. The mean latent times for the high and low tonal stimulations were approximately the same in any given situation. The latencies were reduced by 16% when the subject was required to make a manual discriminatory response, and by an additional 7% under speed response to both tones. In 74% of the discriminatory response records the latent time of blocking was shorter than the manual reaction time. The cortical patterning correlated with "mental set" to make discriminatory and speed responses operated to reduce the blocking time to tone and to increase the effectiveness of tone in altering the brain potential rhythm.