Rice (Oryza sativa L. cv. Safari) plants were grown over a 21-d period in nutrient solutions containing Mn concentrations varying between 0.125 and 32 mg·L-1. The plant shoots and the thylakoid membranes showed similar overall Mn increases for this range of treatments, but different accumulation kinetics. It was found that the highest Mn treatment were associated to the synthesis of a new thylakoid protein with an average molecular mass of 36.5 kDa and a Mn:protein ratio of about 1. This protein exhibited superoxide dismutase activity, as well as a high content of Gln, Asp, Glu, Leu and Gly. Its EPR spectrum is characteristic of high-spin Mn(II), in a S=5/2 ground state. A comparative study of SDS polyacrylamide gel profiles of thylakoid polypeptides from the various treatments disclosed quantitative changes, as well as a new 37/36-kDa polypeptide band in the two highest Mn treatments. The photosynthetic electron transport rates coupled to PSII and PSI showed a significant increase until the 8-mg·L-1 Mn treatment. The related superoxide production of thylakoids (monitored by EPR spectroscopy) showed minimum values from the 0.5-mg·L-1 Mn treatment onwards, which, as shown by the thiobarbituric acid reaction was coupled to a non-significant variation of the acyl lipid peroxidation. It was concluded that Oryza sativa L. cv. safari has a high internal tolerance to Mn as it synthesises a new manganese protein that mimics superoxide dismutase functioning.