Objective - To investigate the role of stromelysin (MMP-3) activity in synovial fluid (SF) at different stages of development and in common joint disorders in the horse. Methods - Stromelysin activity was determined with a fluorogenic enzyme activity assay in SF of normal joints of fetal, juvenile and adult horses, and in SF of horses suffering from the developmental orthopaedic disease osteochondrosis (OC) or osteoarthritis (OA). Additionally, MMP-3 activity was expressed as a ratio of previously reported general MMP activity in the same SF samples. Results - The levels of active stromelysin were 30-fold to 80-fold higher in SF from fetal horses than in SF from juvenile and mature animals (p<0.001). Juvenile horses (5 and 11 months of age) showed a twofold to threefold higher stromelysin activity than adult horses (p<0.05). In OC joints, stromelysin activity was not significantly different from the activity in normal, age matched, control joints. In OA joints the activity was about four times higher than in normal joints (p<0.001). The ratio MMP-3 activity/general MMP activity did not change with age in normal, healthy joints. This ratio was more then twofold increased in OA joints compared with normal joints, indicating selective upregulation of gene expression or activation of proMMP-3, or both, in OA pathology. Conclusions - The significantly higher stromelysin activity in young individuals parallels the higher metabolic activity occurring at rapid growth and differentiation at early age. In OC, MMP-3 mediated matrix degradation appears to be not different from normal joints. The increased stromelysin activity in OA joints is in agreement with pathological matrix degradation. In these joints MMP-3 activity is selectively increased compared with normal joints.