Recognition of capillary endothelial cells was facilitated by a special staining procedure developed previously. Thymidine-labeling indices for endothelial cells were estimated in a number of unstimulated mouse tissues and were found to be in the range 0 to 2.4%. There was no increase in mean labeling index up to 3 weeks after 2000 rads or up to 2 weeks after 4000 rads irradiation of muscle, skin, or bone. After fracture of the femur, the labeling index of capillary endothelial cells in regenerating callus increased to about 10% at 3 to 5 days after fracture and decreased to zero by Day 16. The duration of DNA synthesis in the fractured femur was estimated in a double-labeling experiment to be 7 ± 2 hr. The results of a repeated labeling experiment suggested a turnover time of 80 ± 25 hr on the 6th day after fracture, with a wide distribution of intermitotic times about the mean value. At 3 days after injury, when the labeling index is higher, the corresponding estimate of turnover time is about 50 hr: this value is close to a previous estimate of turnover time for capillary endothelial cells of a mammary tumor growing in the same strain of mouse.