Male and female rats ranging in age from birth to 26 months, were used to study age changes in the rat thyroid and pituitary thyrotrophs. The thyroid follicular cells were cuboidal in shape until 10 mo. of age. Between 10-15 mo. the follicular cells became columnar in shape and contained intracellular colloid droplets. After 15 mo. the gland appeared mixed and consisted of both dilated follicles lined with flattened epithelium and hyperplastic follicles. The pituitary thyrotrophs were not demonstrable at birth but were visible at weaning. The size of the thyrotrophs correlated with the morphological status of the thyroid. Cytoplasmic vacuoles were first evident at 10 mo. and became more prominent as age progressed. The presence of large vacuolated thyrotrophs appeared to be related to the senile hyperplastic thyroid. No sex differences were noted.