Riboflavin enhances photodegradation of bilirubin and might improve the efficiency of phototherapy in hyperbilirubinaemic newborns. Since flavins must be regarded as active photosensitizers, the problem of side effects arises. Therefore, we investigated in Gunn rats dose-response-relationships and side effects. The rats were injected with different doses (from 10 mg/kg up to 200 mg/kg) of the flavin mononucleotide riboflavin-5'-phosphate (r-ph) and illuminated with different effective irradiances (0.3-4.5 mW/cm2). During the first hours after the flavin injection serumbilirubin decline was enhanced in relation to the light- and the r-ph-doses. Histochemical investigations of the Purkinje cells in the cerebellum revealed a significant protection from kernicterus by phototherapy with the high irradiances but no distinct influence of r-ph. Beside reversible inflammatory skin reactions which occurred with high r-ph and medium light doses, massive phototoxic reactions (i.e. large vesicles and necroses) developed on paws, tails and ears when the highest irradiance was used and as an adjunct 50-200 mg/kg r-ph.