A diurnal melatonin rhythm in primate cerebrospinal fluid.

Melatonin was measured in cerebrospinal fluid (CSF) withdrawn continuously from partially restrained rhesys monkeys. There was a daily rhythm in CSF melatonin with peak night values 2- to greater than 15-fold higher than day values. The increase occurred shortly after lights were turned off, and the decrease occurred soon after lights were turned on. There was substantial variation in the magnitude of the rhythm among animals. However, there was little day to day variation in the rhythm of individual animals studied for 3 or 6 consecutive days. Although the concentration of melatonin in CSF was lower than that in plasma, the changes in CSF melatonin concentrations seemed to reflect large daily changes in plasma melatonin concentrations.