40 rats with subcortical lesions and 22 control animals were trained to discriminate the odors of anise and creosote. The operated rats showed no loss of learning ability or retardation in learning. No single central nervous structure was apparently essential for the habit, and mass action, either in the cortex or subcortex, did not operate in the habit. The most likely explanation of the nervous mechanisms underlying the habit was that there were 2 or more equivalent sub-cortical complexes capable of mediating the habit, and upon the disturbance or destruction of one of them the function was carried out by another. At the present time it is impossible to delimit the particular subcortical complexes which may be involved.