The amino-acid precursors tryptophan and tyrosine, and the major metabolites 5-hydroxyindoleacetic acid, indoleacetic acid, homovanillic acid and 3-methoxy-4-hydroxyphenethyleneglycol, related to the central neurotransmitters serotonin, dopamine and norepinephrine, were measured in 62 samples of cerebrospinal fluid from human neonates. Means are reported for the samples from 17 medically uncomplicated infants and for the larger group (42 to 45) of infants with medical complications. The latter group was divided according to diagnosis and medication. All groups had significantly higher levels of all compounds in comparison with older children and adults. There were few significant subgroup differences in the group with complications. In both the normal and complicated groups a number of significant correlations were observed between the compounds themselves and with other physiological measures.