Nitrate reductase activity (NRA) was measured in leaves, stems, roots and nodules of four cultivars of Pisum sativum L. supplied with 2 mM KNO3 and nodulated by selected Rhizobium leguminosarum strains. As a control, non-nodulated (uninoculated) plants were grown in the same nutrient medium. NRA was determined by an in vitro assay; it varied widely among strains and cultivars. Although differences in tissue NRA in nodulated plants were mainly related to cultivar genotype, nodulation much affected the distribution of NRA within the plant. There was also some cultivar x bacteria strain effect. Non-nodulated plants had higher leaf and root NRA than nodulated cultivars, but nodulated plants had twice as high stem NRA as non-nodulated plants. These differences correlated with nitrate concentrations which were depressed in leaves and roots, and increased in stems, of nodulated plants Nitrate accumulation was increased in leaves, stems and roots of nodulated plants in comparison with non-nodulated ones. There were no differences in the root to shoot ratio between nodulated and non-nodulated plants. The relationships between tissue nitrate reduction and nodulation are discussed.