Abstract A rapid, economical method is described for determining between 10 femtomoles (10 −14 mole) and 100 nanomoles (10 −7 mole) of NADH or FMN. The technique involves the measurement of light produced by a bacterial luciferase enzyme. This is conveniently done in a modern liquid scintillation spectrometer with the photomultipliers switched out-of-coincidence. Various factors which affect the assay of NADH and FMN in biological samples have been examined in detail. The bacterial luciferase may be coupled to enzymes utilizing or producing NADH or FMN to produce dynamic measurements in situ.