The presence of adenohypophyseal hormones in human cerebrospinal fluid (CSF) has been firmly established by radioimmunoassay procedures. The hormones can originate from the adenohypophysis itself and from brain, as well as from peripheral sites of production. These peptide hormones enter the CSF through the choroid plexus or by direct secretion. The hormones entering by direct secretion into the CSF have a higher CSF-blood ratio than the hormones entering through the choroid plexus. Elevated CSF levels of adenohypophyseal hormones are most likely to occur in the presence of hormone-producing pituitary tumors with suprasellar extension. Although the CSF concentrations of adenohypophyseal hormones may merely reflect their blood concentrations, it is more likely that these hormones also have paracrine as well as endocrine effects.