The molecular mechanism to control the oxidative burst exerted by Mn accumulation in rice (Oryza sativaL.) plants, grown in hydroponics containing 0.5, 2 and 8mgl(-1) Mn and irradiated with a total biological effective UV-B irradiation of 20.825kJm(-2), was investigated in the chloroplasts at the 15th and 21st days after germination. In both experimental periods, Mn accumulation kinetics in the leaves and in the chloroplast lamellae displayed overall increases. Coupled to higher membrane selectivity, superoxide production and acyl lipids peroxidation in the thylakoids decreased, prompting upper rates of the Hill and Mehler reactions. Connected to UV-B irradiation, higher accumulated Mn in thylakoids was found to be chelated in a 36.5 kDa protein, with Mn/protein ratio of about 1 and high content of Gln, Asp, Glu, Leu and Gly, being its EPR spectrum characteristic of high-spin Mn(II), in a S=5/2 ground state. As this protein exhibited enzymatic catalysis of superoxide dismutation, it was concluded that, under UV-B irradiation, the high internal tolerance of Oryza sativa L. to Mn during the vegetative growth also triggers the synthesis of a manganese protein that mimics superoxide dismutase functioning, therefore furnishing an additional intimate protection against oxidative stress.