Hamsters acquire the ability to effectively regulate their body temperatures in a cold environment (5°-8°C) at ages ranging from 30-44 days, Myelination in the dorsolateral areas of the caudal hypothalamus, as determined photometrically, reaches its height at the time thermoregulation becomes fully developed, It appears therefore that myelination of hypothalamic neurons may be essential to functional maturation of the hypothalamus. The data with respect to hamsters are similar and lend support to those previously reported for albino rats.