Removal of pituitary gland results in atrophy of the thymus. As the pituitary gland is under the control of the hypothalamus, destruction of the anterior portion of the hypothalamus (AHTL) is expected to negatively influence the thymic function. Contrary to our expectation, the thymus became hypertrophic and the serum level of the growth hormone (GH) markedly increased, when the anterior portion of the hypothalamus was widely destroyed in rats at 1 month and over. The results suggested that AHTL removed the cells secreting GHRIH (growth hormone release inhibitory hormone), but not GHRH (growth hormone releasing hormone), leading to increased secretion of GH from pituitary gland and thymic hyperplasia. In other words, the development and aging of thymus appears to be under the balance of the positive (GHRH) and negative (GHRIH) signals of the hypothalamus. It is most likely that the positive signal is high just after the birth and decreases thereafter with a concomitant increase of the negative signal, leading to the onset of thymic atrophy at around puberty.