To determine whether atrial natriuretic factor (ANF) is detectable in human cerebrospinal fluid (CSF), I measured ANF in CSF samples obtained from subjects undergoing myelography for various neurological disorders. Immunoreactive ANF, measured by RIA, ranged from undetectable (less than 9.7) to 36 pmol/L. All measurable samples diluted in parallel in the RIA. Reverse phase high pressure liquid chromatography and subsequent RIA confirmed the presence of ANF, with most of the immunoreactivity eluting at the position of standard human ANF-(99-126). Simultaneous plasma ANF levels were measured in most individuals and the plasma concentrations were less than those in CSF; most of the immunoreactivity in plasma also eluted at the position of human ANF-(99-126). Thus, ANF is present in CSF in substantial amounts in some subjects with neurological disorders. Further, the CSF levels exceeded the circulating plasma ANF levels in most subjects. These results suggest that ANF in CSF is at least partially derived from production in the brain and/or is concentrated in the CSF.