Growth hormone (GH), prolactin (PRL) and gonadotropins (LH and FSH) of cerebrospinal fluid (CSF) and serum were radioimmunologically measured in basal condition in 75 cases and during the stimulation with releasing hormones in 24 of cases, i.e. with TRH plus LRH i.v. Samples were obtained from 75 patients of the following groups: A. control group of 9 patients without endocrine diseases; B. 8 patients with nontumoral diseases, of whom 4 with organic cranial diabetes insipidus and 4 with empty sella syndromes; C. 36 patients with invasive tumors i.e. 6 hypothalamic metastatic carcinomas and 300 invasive pituitary adenomas; D. 22 patients with enclosed tumors, i.e. 1 suprasellar dermoid cyst and 21 enclosed pituitary adenomas. Minimum 3 pituitary hormones were measured in each analyzed sample, and only those hormones, not secreted by the tumoral cells, were analyzed for their differential penetration into the CSF. An increase of the permeability of the blood-CSF barrier (BCB) was frequently observed in the group of invasive tumors and occasionally in the other tumoral or non-tumoral groups. The serum/CSF ratios of gonadotropins become significantly lower than the ratios of GH and PRL in the invasive groups (p less than 0.02), and here the CSF level was correlated to the increases of the serum levels during LRH plus TRH i.v. test only for gonadotropins. These data suggest that the specificity of BCB is not lost, but altered by the tumors of the hypothalamo-hypophyseal region, with an increase of the permeability for gonadotropins more than for GH or PRL.