Neonatal thyroxine administration, behavioral maturation, and brain growth in mice of different brain weight.

L-thyroxine in amounts of .5, 1, and 2 mug was injected daily from age 5 through 14 days in 3 lines of mice selected for high (H), medium (M), or low (L) brain weight. Controls (injected with vehicle only) of these lines differed in ages of eye-opening, first auditory startle, and perfection of a surface-righting response. Thyroxine at .5 mug/day accelerated the maturation of eye-opening, auditory startle, and aerial-righting response, but had no effect on a grasping response or open-field activity. Brain weights were depressed significantly in all thyroxine treated groups except in the 9k-mug dosage in H mice.