To study the role of the hypothalamus in the regulation of growth hormone (GH) synthesis, the rate of amino acid incorporation into GH in vitro was examined in rats with anterolateral deafferentation of the medial basal hypothalamus. Amino acid incorporation into GH (GH synthesis) decreased significantly 7 days after the deafferentation, although prolactin synthesis did not show any significant fluctuations. The serum GH was increased, while the pituitary GH content was decreased. Somatostatin in the stalk median eminence of such deafferentated animals decreased markedly, while GH-releasing hormone was decreased slightly but not significantly. Immunohistochemical examinations revealed that the number of somatostatin nerve terminals in the median eminence decreased markedly, while GH-releasing hormone nerve terminals in the same area remained intact. These findings suggest that neural factor(s) outside the medial basal hypothalamus plays an important role in the regulation of GH synthesis.