Nodulated lucerne plants (Medicago sativa L. cv. Aragón) were supplied with 20 mM nitrate. Anaerobically isolated bacteroids of Rhizobium meliloti from these plants were able to denitrify after 48 h treatment. R. meliloti bacteroids behave as total denitrifiers, reducing nitrate to dinitrogen: when acetylene was omitted from the assay medium very little nitrous oxide was recovered. The onset of denitrification activity was coincident with the induction of nitrite reductase activity (EC 126.96.36.199) whereas nitrate reductase activity (EC 188.8.131.52) was constitutive. Whole nodules from plants receiving several doses of nitrate were assayed, in a nitrate-free medium, to monitor denitrification activity dependent on nitrate within the nodules. Denitrification activity was detected after 2 days of 20 mM nitrate supply or after 3 days in the presence of 10 or 5 mM nitrate. These results are discussed in relation to current controversy about nitrate entry into the infection region of nodules. It is concluded that this process occurs more rapidly than suggested in recent research.