Mesenteric lymph nodes of normal young pigs were perfused in vitro at physiological temperature. Cell-free perfusion medium was pumped into the artery for more than 2.5 hr, and lymphocytes were continuously released into the venous effluent. Recirculating lymphocytes emigrate from pig lymph nodes by entering the blood vasculature directly and not via efferent lymphatics. The presence of lymphocytes in paracortical venular walls after 2 hr of perfusion with new medium suggests that these are the sites of emigration. The rate of emigration of lymphocytes from mesenteric lymph nodes was estimated to be 6 X 10(7)/hr. Study of the lymphocyte populations emerging from the perfused lymph nodes showed that B lymphocytes and E-rosette forming T lymphocytes, but almost no Null lymphocytes, are involved. While the proportion of B lymphocytes remained constant during the perfusion period, E-rosette forming lymphocytes increased significantly. Lymphocyte subpopulations differ profoundly in their capacity to migrate through lymph nodes.