The effect of Equine Hypothalamic Extract (EHE) on pituitary weight and secretion of TSH, FSH-LH and ACTH was studied in the rat. The pituitary response to EHE was assessed by measuring 131I uptake by the thyroid and by weight changes of the pituitary glands, thyroids, adrenals, ovaries and uteri. 131I uptake of the thyroid and weights of the pituitary, thyroid, adrenal and uterus increased in the treated rats, whereas ovarian weights were similar to control groups. These findings indicate that the EHE containes hypophysiotropic peptides which can stimulate the secretion of pituitary hormones in the rat.