The production of localized deep hyperthermia by the radio frequency wave magnetic-loop induction method has been utilized to study the effects of deep hyperthermia on lymphocyte traffic in sheep in vivo. Deep hyperthermia has been applied both to primary peripheral lymph node drainage areas alone (popliteal and prefemoral) and directly over the study lymph node (popliteal). Deep tissue core temperatures were monitored in all studies and the findings were correlated with alterations in the levels of lymphocyte outputs into study node efferent lymph and the volume of efferent lymph flow. In all of 32 studies, there was a prompt and sharp increase in the output of lymphocytes into efferent lymph at 40-43 degrees C. Efferent lymph flow also was increased promptly but to a lesser degree than the output of lymphocytes. The two were not closely correlated. High deep temperature in the nodal area (45 degrees C) appeared to cause delayed nodal malfunction, which depressed lymphocyte output but did not affect lymph flow. Localized deep hyperthermia is an effective and noninvasive means for the local increase of lymphocyte traffic and lymph flow and may prove to be a useful means for the evaluation of the immunological consequences of altering both deep tissue temperature and lymphocyte traffic.