The in vivo rates of tyrosine and tryptophan hydroxylation were studied in three regions of young adult rat brains at 4 times the light-dard cycle. The procedure utilized was to analyze the accumulation of Dopa and 5-HTP after injection of a centrally effective L-amino acid decarboxylase inhibitor, NSD 1015. Monoamine levels were also determined in all control animals and some treated animals. The reate of tyrosine dydrosylation in the telencephalon was significantly higher 7 hrs after dard onset than at the other three times tested. Smaller variations in tyrosine and tryptophan hydroxylation rates as a function of time of day were also observed. 5-HT levels were significantly higher during the light phase than the dark in the telencephalon with the same trend occurring in the diencephalon and brainstem. NA was stable in the telencephalon but reached lower levels in the light and higher levels in the dark in the other two regions. In the telencephalon DA reached high levels early in the light and in the dark phases, showing a biphasic variation. Of particular interest was the apparent lack of carrelation between cyclic changes in the monoamine levels and the changes in hydroxylation rates. Rates of hydroxylation can be considered indicative of rates of monoamine synthesis. This observation is discussed in relation to feedback and other mechanisms regulating synthesis and release of monoamines.