The in vivo rate of brain tryptophan hydroxylation was determined through 
5-hydroxytryptophan accumulation (5-HTPacc) following the administration of NSD 
1015, a L-aromatic amino-acid decarboxylase inhibitor. This measurement was 
performed every 4 h throughout a 24 h hour period in 10 discrete brain areas of 
rats maintained on a regular 12 h/12 h light-dark cycle. The concentrations of 
5-hydroxytryptamine (5-HT) and 5-hydroxyindoleacetic acid (5-HIAA) were also 
determined in untreated rats. Daily variations in 5-HTPacc were found in all the 
areas studied, the 5-HTPacc being higher during the dark period in most 
structures. These results strongly suggest that tryptophan hydroxylation is 
involved in the control of the 5-HT biosynthesis circadian rhythm. However, 
various patterns of 5-HT and 5-HIAA daily variations were observed, suggesting 
that the circadian factors affecting serotonin metabolism can be different among 
brain areas.