The effects of both early mobilization and immobilization on the healing of a crush injury in the rat gastrocnemius muscle were studied histologically. A preproducible injury was induced on the left calf of 242 rats. The histological observations were made 2 to 42 days after the injury and the intensity of the changes was estimated semiquantitatively by scoring. During the first week, the histological changes were especially dependent on the post-traumatization treatment. The inflammatory reaction and haemotoma formation were more pronounced in the mobilized muscles and the amount of necrotic and degenerated muscle tissue was more in the immobilized muscles. The disappearance of the haematoma and of the inflammatory cells occurred more rapidly with mobilization. Scar formation was more pronounced in the mobilized injuries. immobilization for the first 2-5 days after the trauma was followed by aslight delay in the maturation and contraction of the fibrous scar. Muscle regeneration was more extensive and occurred more rapidly in the mobilized muscles. The penetration of regenerated muscle fibres throughh the scar was more pronounced in the immobilized muscles, but the orientation of the new fibres was more parallel with the original muscle fibres after mobilization. The histological differences between the treatment groups diminished markedly towards the end of the observation period.