['The purpose of this study was to compare the expression levels of p65 and S100 in the rat masseter muscle after the injection of different concentrations of botulinum toxin-A (BTX-A).', 'We injected either 5 or 10\xa0U of BTX-A into both masseter muscle of rats. As a control group, the same volume of saline was injected. After 14\xa0days, the animals were sacrificed. Subsequently, a biopsy and immunohistochemical staining of the samples were performed using a p65 or S100 antibody.', 'The cross-sectional area of each myofibril was significantly reduced by BTX-A injection ('] Are s100 and p65 expression increased in the masseter muscle after botulinum toxin-A injection?