This protocol evaluates measures of serotonergic (5-HT) function in psychiatric or developmental disorders. The responsivity of CNS 5-HT pathways is assessed by neuroendocrine challenge testing (fenfluramone challenge test) in adolescents and young adults. Children and older youngsters donate blood for measurement of platelet 5-HT receptor binding indices and responsivity, whole blood and plasma 5-HT content, tryptophan levels, and other neurochemicals.