A kinetic study of Drosophila melanogaster sepiapterin reductase has been done. The apparent K m for sepiapterin has been estimated as 153 μM. N-acetyldopamine, N-acetylserotonin, andN -methoxyacetylserotonin have shown to be noncompetitive inhibitors for the Drosophila sepiapterin reductase. The K i obtained were 40 μM, 127 μM and 87 μM, respectively. Chromatofocusing of Drosophila crude extracts has shown only one peak with sepiapterin reductase activity, corresponding to pI 6.0. A study of the activity in different eye-color mutant strains revealed significantly decreased levels in clot, red Malpighian tubules and garnet.