A comprehensive study of lymphocyte traffic in AO rats was performed under conditions as near to the physiological state as was practicable. In the light of previous results on the effect of environmental factors on the migratory behaviour of lymphocytes, thoracic duct cells were passed from blood to lymph in an intermediate rat before injection into a series of recipients for examination at time intervals from 1 min to 24 hr. At 1, 2 and 5 min after injection most of the labelled cells were in the blood, lungs and liver. The concentrations in these compartments fell over the next 25 min as the cells entered the spleen, lymph nodes (LN) and Peyer's patches according to a regular pattern. The peak localization in these latter organs occurred between 1 hr and 18 hr. Each organ had a characteristic time pattern of lymphocyte localization. Entry into mesenteric LN lagged behind other LN until 2.5 hr after injection following which mesenteric LN localization easily outstripped that in other LN to reach a delayed peak at 18 hr. Intravenously injected lymphocytes began to reappear in thoracic duct lymph in large numbers earlier than in previous studies so that the time taken for most T lymphocytes to cross LN from blood to lymph fell within the broad time band of 4-18 hr. Lymphocytes took on average 5-10 min to cross high endothelial venules when entering LN from the blood.