The interactions between Mn accumulation and production and control of oxy radicals were investigated in rice (Oryza sati6a L. cv. Safari) chloroplasts. Rice plants were grown for 21 days in nutrient solutions containing 2.3-582.5 mM Mn. The plant shoot and the photosynthetic membranes showed 18-fold increases over this range of treatments, but displayed different accumulation kinetics. Production of superoxide and hydroxyl radicals in the thylakoids decreased significantly up to the 36.4 mM Mn treatment, but increased at higher concentrations. In the thylakoid lamellae the total activity of superoxide dismutase (SOD) showed a similar pattern. The enzyme activities of the peroxide system coupled to the photophosphorylation varied significantly with increasing Mn concentrations. Between the 2.3 and the 36.4 mM Mn treatments, the Vmax of ascorbate peroxidase decreased, but thereafter, until the 582.5 mM an opposite trend was found. The activity of dehydroascorbate reductase did not vary significantly with Mn concentration. Between the 2.3 and the 9.1 mM Mn treatments the Vmax of glutathione reductase decreased significantly. In cellular extracts, the Vmax of catalase decreased significantly until the 145.6 mM Mn treatment but in the highest Mn treatment this enzyme activity increased. It was concluded that rice shows a biphasic response to Mn concentrations in the chloroplast lamellae. Until the 36.4 mM, the increasing photosynthetic rates along with the decrease of free radicals production and of the maximum activities of ascorbate peroxidase and SOD indicates that an efficient adaptation of chloroplasts to increasing Mn concentrations occurs. Moreover, from the 36.4 mM onwards, as this process is reversed, the main conclusion is that Mn accumulation starts to be toxic, leading to oxidative stress.