In normal young pigs two labeling procedures were evaluated to study the production of lymphocytes in mesenteric lymph nodes. Tritiated thymidine was injected either into the superior mesenteric artery or directly into the lymph nodes. There was a high rate of lymphocyte production and a considerable export of newly-formed small lymphocytes. The distribution to lymphoid organs and the appearance of labeled lymphocytes in the blood were comparable after both techniques. The intranodal labeling resulted in a more selective labeling and thus was found to be the superior method for quantitative studies.