A case of a 60-year-old man with severe trismus after inferior alveolar nerve block is presented. MRI scans as well as histologic examination revealed muscle fibrosis and degeneration of the medial part of the left temporal muscle due to myotoxicity of a local anesthetic agent. Palabras clave: anesthesia, fibrosis, myotoxicity, temporal muscle, temporomandibular joint, toxicity, trismus