The effect of Indomethacin upon the process of fracture repair and osteogenesis in bone isografts has been studied in the rat. It was found that the drug had no significant effect upon new bone formation in heterotopic bone grafts. It had no effect upon the osteogenesis in repairing drill holes in 2 month old rats. A significant impairment of osteogenesis was detected, however, in older (6--9 months) animals given continuous treatment with Indomethacin from the week before fracture. This effect was not apparent if treatment was terminated on the day after induction of the lesion. There is histological evidence of increased fibrogenesis and decreased osteogenesis and remodelling in fractures in old rats given high dosages of Indomethacin.