The extent of circadian variation in plasma corticosteroid levels at 30 days of age was normal in rats who received either intraventricular 6-OH dopamine or 5,6-dihydroxytryptamine on day 3 of life. A 4-h advance in the time of peaking was present in this latter group. At the time of study the depletion of 8.00 a.m. and 8.00 p.m. levels of norepinephrine was significant in the cortex (–49%), hypothalamus (–58%), hippocampus (–65%), and amygdala (–44%) of the 6-OH dopamine-treated animals, as was a similar depletion of serotonin in these areas in the 5,6-dihydroxytryptamine-treated animals. There was no difference between 8.00 a.m. and 8.00 p.m. levels of either amine in any of these areas, in contrast to the variation seen in uninjected animals in whom 8.00 a.m. levels of both amines were higher in all areas than 8.00 p.m. levels. The female 6-hydroxydopamine-treated animals had significantly increased adrenal weights. No other significant changes were noted in the adrenals or gonads of either treated group. Body weights were not significantly altered in any of the animals. These data demonstrate that the circadian variation in plasma corticosteroid levels can develop in the presence of either marked norepinephrine or serotonin depletion in CNS areas that have been implicated in the regulation of such peri