Concentrations of the neurotransmitters, serotonin (5-HT), dopamine (DA), and norepinephrine (NE) were measured in the developing rat brain at 12, 19, 26 and 42 days of age. The amino acid precursors, tryptophan (TRP) and tyrosine (TYR) were measured along with the 5-HT and DA metabolites, 5-hydroxyindoleacetic acid (5-HIAA) and homovanillic acid (HVA), in brain and cerebrospinal fluid (CSF) at the above ages. This first report of CSF HVA levels in the developing rat shows that it, like 5-HIAA, declines with age. In contrast, the ontogeny of the compounds in brain are dissimilar, with 5-HIAA remaining relatively constant with age while HVA declines markedly. Possible reasons for the differences and similarities in the ontogeny of 5-HIAA and HVA levels in brain and CSF are discussed. The persistence of the ontogenetic pattern for the neurotransmitters and acid metabolites after central DA depletion is also reported.