A total of 210 male Charles River CD rats, 45 days old, were subjected to a fracturing of the right radius and ulna by digital pressure while under ether anesthesia. These animals were then assigned randomly to dose groups (1, 2 or 4 mg/kg/day of indomethacin and 100, 200, or 300 mg/kg/day of aspirin) and were dosed for 21 days. Dose levels were chosen to provide approximately equipotent levels of the test compounds with the highest dose approaching toxicity. Radiographs were taken of control-rat fractures on days 8, 14, and 21. Three rats at 4 mg/kg/day of indomethacin died of interstinal perforation prior to scheduled sacrifice. On day 22, all remaining rats were sacrificed by exsanguination under anesthesia. Histologic secretions of the radius and ulna were examined in random sequence without knowledge of the treatment regimen. A histologic grade based on the morphologic stage of fracture healing was given. There was a drug- and dose-related retardation of fracture healing, which was statistically significant at all dose levels of indomethacin and the highest level of aspirin, as compared to controls. Decreased mean grades in the groups given 100 and 200 mg/kg/day of aspirin, though not statistically significant, suggest a retarding effect on fracture healing at these levels also. No statistically significant changes in numbers of pseudoarthroses were found.