Background Amyotrophic lateral sclerosis （ALS） is a fatal neurodegenerative disorder characterized by progressive death of the upper and lower motor neurons. Transgenic mice over-expressing a mutant form of the human SOD1 gene develop an ALS-like phenotype. Currently, there is no effective treatment or drug for the fatal disease. Previous studies reported potent efficacy of dl-3-n-butylphthalide （DL-NBP） for several neurodegenerative disorders and cerebral ischemia. SOD1-G93A mice are a mouse model of ALS. In this study, we investigated the efficacy of DL-NBP on this ALS mouse model. Methods Sixty SOD1-G93A female mice were divided into four groups. The vehicle control group received 0 mg.kg-1.d-~ DL-NBP. The experimental groups received DL-NBP with doses of 30, 60 or 120 mg.kgl.d1, respectively. For measurement of motor activity, the hanging wire test and rotarod test were performed. Survival statistics were analyzed by Kaplan-Meier survival curves. The body weight of each mouse was recorded twice per week. The statistical motor unit number estimation （MUNE） technique was used to estimate the number of functioning motor units in gastrocnemius muscle. Muscle morphology was evaluated by hematoxylin and eosin staining. Motor neuron quantJtation was performed by Nissl staining and microglia activation was observed by immunohistochemistry. Results Oral administration of 60 mg.kg-l-d-1 DL-NBP significantly prolonged survival （（164.78±16.67） days） of SOD1-G93A mice compared with vehicle control （（140.00＋16.89） days）. Treating mice with DL-NBP （60 mg.kg-1.d-1） significantly decreased the progression rate of motor deficits and suppressed body weight reduction. Furthermore, we found that treating SOD1-G93A mice with DL-NBP （60 mg.kgl.d1） slowed the rate of MUNE reduction （P 〈0.01）. Motor neurons were remarkably preserved in the anterior horns in mice treated with DL-NBP （60 mg.kg-1d-1） at the stage of 19 weeks （P 〈0.01）. Treating mice with DL-NBP （60 mg.kg1.d1） si
