A rabbit antiserum to somatostatin was used to develop radioimmunoassay methods for measuring somatostatin in monkey cerebrospinal fluid (CSF). By gel permeation chromatography, at least five molecular weight forms of immunoreactive somatostatin (IRS) were identified in monkey CSF; two of these species co-migrated with either synthetic somatostatin-14 or somatostatin-28. The 24-hr profile of CSF somatostatin immunoreactivity was obtained from five monkeys during diurnal lighting, constant light, and constant darkness. During diurnal lighting, all five monkeys had a clear ultradian component in CSF IRS of 4 to 5 hr duration; this pattern was not affected significantly by constant light or dark. In addition, there of the five monkeys exposed to diurnal lighting showed a diurnal rhythm in CSF IRS, with higher hormone levels during darkness. In some animals, this diurnal rhythm also could be demonstrated during constant light or dark.