Changes in the protein content, maximal activity, and K m of phosphate-dependent glutaminase were measured in the lymphoid organs (thymus, spleen, and mesenteric lymph nodes) from just-weaned, mature (3 months), and aged rats (15 months). Also, [U 14C]glutamine transport and decarboxylation and the production of glutamate and aspartate from 2 and 20 m M glutamine were measured in incubated mesenteric lymph node lymphocytes. The ageing process induced a reduction in the protein content of the thymus and spleen, as well as the phosphate-dependent glutaminase activity in the thymus and isolated lymphocytes. The K m of phosphate-dependent glutaminase, however, was not affected by the process. Ageing reduced [U- 14C]glutamine decarboxylation and increased glutamate and aspartate production in incubated lymphocytes. The results indicate that the ageing process does modify several aspects of glutamine metabolism in lymphocytes: reduces maximal glutaminase activity and [U- 14C] glutamine decarboxylation and increases the K m for [U- 14C] glutamine uptake and the production of glutamate and aspartate.