The proliferation rate of vascular endothelium has been studied using repeated administrations of tritiated thymidine, given every 8 h for 1 week. Five experimental mouse tumours have been investigated and compared with placenta and with normal tissues. The large difference in labelling indices between tumour and normal endothelium that has previously been detected with single injections of ([3H]dT) is confirmed by these continuous labelling studies. The potential doubling time of the tumour endothelium is estimated to be between 2.4 and 13 days for the five tumours. Tpot for the placenta is at least as short. The turnover time of the normal tissue endothelium is estimated to be 20-2000 times longer (47-23,000 days) and does not seem to differ in slow turnover tissues e.g. lung and liver from that in tissues where the parenchymal cells are rapidly turning over e.g. jejunum or skin.