We induced apoptosis in normal rats by intramuscular injection of 5-fluorouracil (5-FU) and immunohistochemically evaluated the thymus for the TdT-mediated dUTP biotin nick end labelling on the 1st, 3rd, 6th, 9th, 15th and 21st days following the bolus intramuscular injection. The injections of 5-FU induced a greater extent of apoptosis in the thymus. In the thymus, a mild increase in apoptosis was observed 24 hours after injection. The greatest number of apoptotic cells were seen at 72 hours. The size of the thymus decreased and the cortex thinned with hypocellularity. The injection of 5-FU caused massive cell loss in the thymus. Most apoptotic cells were scattered in the cortex and lower levels of apoptosis were also observed in the medulla. After 72 hours, the level of apoptosis returned to the control level. Considering the above results, we think that 5-FU induced toxicity may be related to 5-FU induced apoptosis in normal tissue, especially the thymus.