An assay of reduced nicotinamide adenine dinucleotide phosphate (NADPH) by bioluminescence was investigated and applied for NADP+. The NADP+ is first reduced by glucose-6-phosphate dehydrogenase and then assayed in a mixture containing a NADPH/flavin mononucleotide oxidoreductase which in turn activates luciferase. Many interferences were observed and the method was modified accordingly. NADP+ and NADPH can be assayed separately or simultaneously within the range 1-100 pmol, which is sufficiently sensitive to be applied to biological materials. Many details and precautions must be taken into consideration.