Thyroxine was administered to infant rats within the first 3 days of postnatal life; controls receiving 0.01N NaOH were from the same litter. Thyroxine accelerated the maturation of the pituitary-adrenal response to elec tric shock. The "startle response" ap peared earlier in the experimental ani mals, as did the development and re sponse of the electroencephalogram to novel stimuli. The thyroxine-treated rats, when 16 to 18 days old, acquired a conditioned-avoidance response faster than did controls.