The cytochrome P-450 dependent monooxygenase system of rat liver microsomes is investigated by light dosimetry (action spectroscopy). The scoparone-O-demethylation activity is enhanced by light and depends on the wavelength of the irradiating light. The relative increase of the activity (about 15%) by the irradiating light (approximately 0.5 mW/cm2) is maximal at a wavelength of 400 nm. The light induced enhancement of the 7-ethoxycoumarin-O-deethylase activity was measured in reconstituted systems, consisting of one of two P-450 enzymes (P-450 beta NF-B and P-450 PB-B) with the NADPH-cytochrome P-450 reductase. The action spectrum of the reconstituted P-450 beta NF-B:NADPH-P-450 reductase complex shows a maximum between 420 and 440 nm. The relative increase of the activity induced by light of 420 nm was 7.3% and 9% for the reconstituted systems of P-450 beta NF-B and P-450 PB-B, respectively. The results are discussed in analogy to the classic experiments of Warburg with its blocked CO-enzyme-complex. The results can best be explained by the assumption that the light induced enhancement of the enzyme activity is due to an excitement of those intermediate states of the P-450 catalytic cycle (ferric and ferrous state of the heme iron) which are rate limiting.