Murine progressive ankylosis (MPA) is an heritable disorder which produces acute arthritis and ankylosis of peripheral and axial joints similar to ankylosing spondylitis. Because indomethacin inhibits heterotopic bone formation in vivo, we studied its effects on MPA. Indomethacin was administered for 6 weeks beginning at weaning to litters from heterozygote breeder pairs. Progession curves for peripheral, spinal and thoracic joint ankylosis in treated and untreated ank/ank animals wer compared. There was no delay in ankylosis of peripheral or spinal joint ankylosis in MPA animals treated with indomethacin. interestingly, transient delay of thoracic ankylosis occurred in indomethacin treated MPA animals.