Brain potentials were recorded from the left occiput of 6 subjects known to have a good a rhythm, while they responded to lists of opposites-words during 10 min. work periods. 5 min. relaxation periods were used as controls. The definition of block is that used by Bills. The % of occurrence of the a rhythm was significantly greater during the control period than during the normal response period or the period of blocked response, but there was no significant difference in % of occurrence during normal response and blocked response. Frequency of the a rhythm was higher during response and block periods than during relaxation periods. No significant difference between response and block periods was noted.