Nitrite production by nodules and roots of pea plants (Pisum sativum L., cultivar Alaska) inoculated with Rhizobium leguminosarum strain 3855 has been studied. Nitrate reductase (NR) activity and nitrite reductase (NiR) activity of the bacteroidal and cytosolic fractions of the nodules were also determined, as well as the nitrite content of the nodules cytosol. Nitrite production by nodules and roots from plants treated with 5 mM KNO3 was higher than that of nodules and roots from plants not treated with nitrate, and regardless of the nitrate treatment, nitrite production increased with the incubation period. The presence of nitrate, propanol or both compounds in the incubation mixtures significantly increased the nitrite production by nodules and roots. Nitrite reductase activity was detected in fresh by isolated bacteroids of R. leguminosarum strain 3855, although the presence of nitrate reductase activity could not be detected both in bacteroids of nodules isolated from plants treated or not with 5 mM KNO3. After isolation, when bacteroids were incubated in a mixture with nitrate, nitrate reductase activity developed after incubation for 12 h. Consequently, there was an increase in nitrite reductase activity, which resulted in the disappearance of the nitrite previously accumulated in the incubation medium. Nitrate utilization by bacteroids was not detected until 5 h from the beginning of the incubation period. Since the presence of chloramphenicol or rifampicin in the incubation medium prevented the development of the nitrate reductase activity, such activity was induced in bacteroids. Nitrite content and nitrate reductase and nitrite reductase activities of the cytosol from nodules of pea plants treated or not with 5 mM KNO3 varied with the buffer used for nodules homogenization. However, no nitrite was found when nodules were homogenized with ethanol, what indicates that nitrite accumulation in the cytosol occurs during the homogenization process of the nodules.