Guinea-pigs were treated with different doses of 5-FUdR and investigated at different intervals from the treatment. The weight of the different lymphoid organs was registered. The release of lymphocytes from the thymus was determined by comparison between the content of lymphocytes in blood from the thymic vein and from the carotid artery. In one group the blood flow through the thymus was determined and the total output of lymphocytes from the thymus per min was calculated and compared to that in normal animals. The results showed that, among the lymphoid organs, the thymus was the one most sensitive to 5-FUdR. No regeneration of thymic weight was seen within 16 days. Both spleen and cervical lymph nodes showed initial atrophy, followed by a rapid recovery. The release of thymic lymphocytes was abolished 2 days after a single dose of 5-FUdR. At 4 days a release of both small and large cells occurred. At 8 and 16 days again no export occurred. The release of lymphocytes after 4 days coincided with the rapid regeneration of the spleen and lymph nodes. It is suggested that the release of premature lymphocytes from the involuted thymus is due to a regulative feed-back mechanism from the non-thymic lymphoid tissue depleted of lymphocytes.