THE CLASSICAL CONCEPT of neuroendocrine control of the pituitary as proposed by Harris stipulates that the endocrine functions of the anterior pituitary are controlled by hypothalamic releasing factors or release-inhibiting factors. It was postulated that these factors are present in hypothalamic neurons which project into the median eminence (ME) of the basal hypothalamus and end in contact with the hypothalamic-hypophyseal portal vessels (1). It has subsequently been shown that growth hormone (GH) secretion is regulated by both stimulatory and inhibitory factors of hypothalamic origin.

Like other anterior pituitary hormones, GH is secreted episodically. In all mammalian species so far studied spontaneous episodes of GH secretion occur several times over a 24-h period (2–8). Particularly in the adult male rat there is a striking regularity in the GH pulses which occur at 3- to 4-h intervals and reach levels of several hundred ng/ml.