• Lumbar cerebrospinal fluid homovanillic acid (HVA), 5-hydroxyindoleacetic acid (5-HIAA), and probenecid were measured in four subgroups of neuropsychiatrically disturbed children and a contrast group of pediatric patients. With the exception of a serotonin metabolite difference between autistic and nonautistic psychotic children, there were no significant differences in metabolite concentrations among autistic, nonautistic psychotic, aphasic, and cognitively and attentionally impaired groups, or between the developmentally disabled and contrast groups of children. Younger children had higher concentrations of HVA than older children. Girls had significantly lower HVA/probenecid ratios than boys, which did not appear to be related to underlying neuropsychiatric disorder. Significant probenecid-metabolite correlations indicate the importance of measuring probenecid in the cerebrospinal fluid in clinical studies.