AbstractLocal calcergy is an ectopic calcification reaction (due to hydroxyapatite formation) induced in connective tissue sites by the application of dilute solutions of certain metallic salts (calcergens). The phenomenon is usually produced in the subcutaneous tissues of the experimental animal. The single intra‐articular 66injection of lead acetate (PbAc) solution (a known calcergen) into the knee joint of the rat is followed by opacity of the synovial membrane and para‐articular tissues with the aggregation of macrophages and multinucleated giant cells and the formation of some granulation tissue. This lesion ultimately resolves and there is no cartilaginous degeneration. Repeated intra‐articular injections of PbAc produce pronounced changes in the synovium and para‐articular tissues. There is extensive calcific deposition with exuberant macrophage and giant cell accumulation and fibroblastic proliferation. The proliferated synovial membrance becomes adherent to articular cartilage. There is thinning and fragmentation of the latter with focal loss and replacement by fibrous tissue. Therefore, in contrast the effects of a single injection, repeated calcergenic stimulation of the knee joint is associated with a destructive arth