In this study, the role of cell locomotion in lymphocyte traffic was investigated following the in vivo fate of 51Cr labelled lymph node cells pretreated in vitro with substances known to modify cell motility. The results obtained show that lymphocyte migration is an energy dependent process, as it could be impaired by sodium azide. That it requires the integrity of microfilaments, as it was modified by Cytochalasin B; and that it is altered by isoproterenol, theophylline, prostaglandin E1 and dibutyryl cyclic-AMP. The relevance of these findings to the process of lymphocyte recirculation is discussed.