A study was undertaken to investigate the healing and regeneration of articular cartilage following trauma. Surgically created superficial and depp lesions involving the articular surface of the femoral condyles were evaluated in 28 dogs at intervals of two four, eight and sixteen weeks. The general conclusion reached was that cartilage resonded to trauma either with fibrous tissue repair (undergoing metaplasia to fibro--cartilage) when the lesion was deep, or by limited cellular replacement before the superficial layers became reorganised into zones resembling the normal, if the lesions were superficial. This process tended to tail off before the defect was filled i.e. the regenerative process appeared to cease before the lesion was anatomically restored to normal.