The effects of thyroid and adrenal cortex hormone and sex hormone deficiencies on the ability to learn, store memory traces, and behave were compared in male rats. These studies showed that removal of peripheral endocrine glands led to disruption of the learning process and the ability to reproduce learned information and also produced alterations in behavior. Analysis of the results showed that corticosteroid hormones are directly involved in the processes of learning and behavior. Sex and thyroid hormones appear to have modulatory effects on higher nervous activity.