Possible causes for tolerance of certain rice varieties to manganese..The physiological explanation for Mn‐tolerance of certain rice‐varieties was surched in a long‐(3 weeks) and short‐termed (24 h) experiment with 4 and 5 weeks old intact rice‐plants. Three rice‐varieties are used which were resp. different in their tolerance to an high level of manganese (long‐termed experiment: IR‐8 + susceptible, IR‐22 + intermediate, IR‐661 + resistant; short‐termed experiment: Peta + susceptible, IR‐22 + intermediate, Ml‐48 + resistant). The rice plants were grown up for 4 resp. 5 weeks under the same conditions with a manganese‐level in the nutrient solution of 1 ppm (MnCl2) and 5 ppm Fe (FeEDTA). Then uptake and translocation of Mn and Fe was studied in the same nutrient solutions with different levels of Mn (0,01‐1 ppm). In the long‐termed experiment toxic symptoms, which resulted from the high Mn‐level, were observed. The following results were obtained: 1. At high manganese levels in the substrate only the shoots but not the roots showed Mn‐induced toxic symptoms. 2. In relation to the extent of tolerance of the rice‐varieties, the high Mn‐level in the substrate resulted in the shoots in chlorosis with following nekrosis, in a reduced number of leaves and a reduced dry matter. 3. Irrespective of differences in variety and Mn‐level, the shoots had, in contrast to the roots, a much more Mn‐ than Fe‐content. Therefore the Mn‐sensitive shoot of rice plants had an high Mn/Fe‐relation, while this relation was low in Mn‐resistant roots. 4. There are no differences in Mn‐uptake and translocation of the three varieties in relation to their extent of tolerance to an high level of manganese. 5. At low (0,01–0,1 ppm) and normal )1‐10 ppm) levels of Mn in the substrate, uptake and translocation of Fe was higher in varieties which are sensitive, than in those wihich are intermediate or resistant to an high Mn‐level. 6. Increasing of Mn reduced Fe‐uptake and ‐translocation, but at toxic levels of Mn in the substrate (50–100 ppm), uptake and translocation of Fe was now higher in varieties which are intermediate and resistant to an high Mn‐level than in those which was Mn‐sensitive. Therefore rice‐varieties which are resistant or intermediate to high Mn‐levels can maintain a much lower Mn/Fe‐relation, than those which are Mn‐sensitive, esp. in the shoots. 7. On this basis of manganese‐induced Fe‐deficiency, and of better Fe‐uptake and ‐translocation at toxic Mn‐levels, Mn‐tolerance of certain rice‐varieties is discussed. .