Plasma and cerebrospinal fluid (CSF) specimens were measured simultaneously for human chorionic gonadotropin (HCG) in two patients with HCG- secreting choriocarcinoma. In the patients with hypothalamic tumors, the CSF HCG levels were higher than the plasma HCG concentrations. In the patient with gestational choriocarcinoma with no known cerebral metastases, the plasma HCG level greatly exceeded the CSF HCG concentration. The finding of a CSF HCG concentration that approaches or exceeds the plasma value would be a useful screening procedure in localizing a pathologic source of HCG secretion in patients with a suspected hypothalamic tumor. An unexpected finding in the patient who also had a hypothalamic embryonal cell carcinoma and hypocortisolism was an extremely high concentration of a biologically inactive adrenocorticotropic like substance in the CSF.