Sham-operated and thymectomized guinea-pigs were treated with 5-FUdR. The release of lymphocytes from the spleen was estimated by comparison between the cell content in afferent and efferent splenic blood. The thymectomized animals had a lower number of circulating blood lymphocytes and a slightly smaller release of lymphocytes from the spleen than the sham-operated animals. The treatment with 5-FUdR reduced the number of circulating blood lymphocytes to about the same extent in sham-operated and thymectomized animals. The splenic release of small lymphocytes to the blood was also decreased by 5-FUdR in both sham-operated and thymectomized animals. Thus, the results indicated that 5-FUdR interfered with the release of lymphocytes from the spleen. No evidence was obtained for a major role of the thymus in the recovery of a normal splenic release of lymphocytes to the blood after treatment with 5-FUdR.