Light has a profound influence on ochratoxin biosynthesis by Penicillia. When incubated under constant daylight of a certain intensity, ochratoxin A biosynthesis is decreased by about 20-30% compared to incubation under constant darkness. Under day/night oscillation, the ochratoxin A polyketide synthase gene, a key gene of the ochratoxin A biosynthesis pathway, is rhythmically expressed, and moreover, the amount of ochratoxin also oscillates between the amounts produced either during constant darkness or during constant light. This indicates a partial degradation of ochratoxin A (20-30%) under light conditions until a certain lower limit is reached. This behavior is dependent on the light intensity. At 1,600 Lux, only weak effects could be observed; however, at 2,800 Lux, the effects became significant. After growth under constant light conditions, Penicillium produced ochratoxin B at amounts which are 5 times higher than after growth in constant dark or in alternating light/dark conditions. Growth experiments in the dark on medium with increasing amounts of ochratoxin A revealed that externally applied ochratoxin is moderately toxic. However, if the same growth experiments are carried out under light conditions, the growth inhibiting activity of ochratoxin A is greatly increased, indicating that light amplifies the toxic activity of ochratoxin. Because of the oscillation of the concentration of ochratoxin A during night and day incubation, Penicillium seems to have developed an adaptive mechanism to reduce the amount of ochratoxin A during daylight below a toxic level.