Partially purified thymosin fraction 5 and one of its synthetic peptide components, thymosin beta 4, but not thymosin alpha 1, stimulated secretion of luteinizing hormone--releasing factor from superfused medial basal hypothalami from random cycling female rats. In addition, luteinizing hormone was released from pituitary glands superfused in sequence with hypothalami. No release of luteinizing hormone in response to thymosin was observed from pituitaries superfused alone. These data provide the first evidence of a direct effect of the endocrine thymus on the hypothalamus and suggest a potentially important role for thymic peptides in reproductive function.