Fluridone, an inhibitor of the carotenoid biosynthesis, was used to study the relationship between the degree of carotenoid depletion and the function of the photosynthetic apparatus. The data reveal that, at a small reduction of the carotenoid content (25% decrease of the total carotenoids), the PSII and PSI (oxidation of P700 by far-red light) photochemistry is not influenced, while the oxygen evolution is strongly inhibited. Further reduction of the total carotenoid content (more than 40%) leads to decrease of the chlorophyll content and inhibition of the functions of both photosystems as the effect on the photosynthetic oxygen evolution and primary photochemistry is stronger than the effect on P700 oxidation. The analysis of the oxygen production under continuous illumination and flash oxygen yields suggests that the inhibition of the oxygen evolution is caused mainly by the damage of PSIIalpha centers.