The physiological process of lymphocyte migration is a complex and dynamic process. The differential migration and life span of lymphocyte subsets is inherent to the normal function of the mammalian immune system. Adequate assessment of the involved processes requires the presence of an intact blood and lymphatic circulatory system and the ability to isolate individual tissues. The sheep provides an invaluable experimental model for studying these processes. Recent data suggest that direct quantitation of the life span of individual subsets of recirculating memory and naive lymphocytes is now possible, and that the long-term characterization of the behaviour of recirculating cells can be undertaken. Finally, it appears that previous qualitative data on tissue-specific homing pools can now begin to be understood in the context of phenotypic analysis for T cell markers and adhesion molecules, combined with long-term tracking techniques.