A complete clinical study of a hypothyroid boy, age 13, was made for a period of 2 years. An examination of the higher nervous activity by the conditional reflex method showed that the ability to form and differentiate conditional reflexes ran parallel to the metabolic records (BMR, serum cholesterol) and to the EEG, showed a marked improvement within several weeks after thyroid therapy, the improvement in these items reaching a level in 2 months, without further change after 2 years of therapy. The IQ (depending largely upon an accumulation of learning) showed a marked stability over the same 2-year period. The conditional reflex, on the contrary, is a delicate measure of higher nervous function practically independent of education, related specifically to the status praesens of the patient and in this hypothyroid patient to basic metabolic changes.Abstract Teaser