Catecholamine- and serotonin-containing neurons were studied inpale, aDrosophilathird-chromosome recessive lethal mutant. Using histofluorescent and immunocytochemical techniques, we show that this mutation only alters catecholamine levels in the CNS. Both the presence of catecholamine-neurons and the expression of serotonin are not affected by the mutation. Furthermore, we show that normal characteristics of catecholamine neurons, such as the presence of the enzyme DOPA decarboxylase and the selective uptake properties are normal inpalemutants. We suggest thatpaleis either the tyrosine hydroxylase structural gene, or a gene controlling tyrosine hydroxylase activity inDrosophila. The similar genetic location of the putative tyrosine hydroxylase gene and the mutationpalesupports the former suggestion.