Cerebrospinal fluid was continuously collected from the cisternal-cervical subarachnoid space of chair-restrained rhesus monkeys. The concentrations of melatonin and cortisol were measured in the cerebrospinal fluid. Under diurnal lighting (light:dark, 12:12 h) melatonin concentrations were elevated during darkness and low during illumination. The melatonin rhythm persisted in constant darkness but was suppressed in constant illumination. Under diurnal lighting, cortisol concentrations were elevated in the early portion of the light period. This daily rhythmicity of cortisol secretion was not altered by constant illumination or constant darkness. The differential response of the two hormones to constant light suggests that the daily fluctuation of melatonin secretion was not responsible for the daily rhythmicity of cortisol secretion in the rhesus monkey.