One ml of 0.106%, 0.425% or 1.7% acetic acid (AA) solution, isotonic to physiological saline solution, was injected into the M. vastus lateralis of male Japanese white rabbits. Two, 7, 14 and 21 days after the injections, the rabbits were autopsied, and the removed muscles were observed macroscopically. After fixation with 10% neutral formalin, they were examined histologically. Hemorrhage and white degeneration of the muscles were grossly observed in 0.106%, 0.425% and 1.7% AA at 2 days. These lesions disappeared at 7 days and thereafter in 0.106% AA, and at 14 days and thereafter, except for one animal, in 0.425% AA. In 1.7% AA, these lesions were observed until 21 days, but gross hemorrhage began to recover at 21 days. The size of the lesion, which was expressed as a product of the major axis (mm) and minor axis (mm) of the lesion, showed reduction over time, especially in 1.7% AA. Histologically, swelling, necrosis and calcification of the muscle fibers and cell infiltration, hemorrhage and dilatation in the interstitium were observed after the injections of 0.106%, 0.425%, and 1.7% AA. At 7, 14 and 21 days, regeneration of the muscle fiber and fibrosis in the interstitium were observed. The injected sites were examined by injecting a chip of test-tube brush as a marker. In 20 (83%) of 24 cases, it was ascertained that the markers were injected into the M. vastus lateralis. The markers were found in the M. vastus intermedius in 3 out of 4 other cases and in the intermuscular tissue between the M. vastus lateralis and M. tensor fasciae latae in one case.