Abstract: Objective:To investigate the expressions and spatial distributions of myosin heavy chain-Ⅱ(MHC-Ⅱ),myosin heavy chain-Ⅰ(MHC-Ⅰ) and -smooth muscle actin(-SMA) in muscular layer of esophagus in the healthy adult dogs.Methods:The expressions of MHC-Ⅱ,MHC-Ⅰ and -SMA in muscular layer of esophagus in 6 healthy adult dogs were detected by histology and immunohistochemistry.Results:The expressions of MHC-Ⅱ and MHC-Ⅰ in muscular layer of whole esophagus in adult dogs were detected.The expression intensity of MHC-Ⅱ gradually decreased from the head to end of esophagus,which was still strong in cardiac muscle layer around the lower esophagus.The expression intensity of MHC-Ⅰ gradually increased from the head to end of esophagus,which was still strong in cardiac muscle layer around the lower esophagus.The little -SMA expression began in the muscle layer of the middle-lower esophagus,which gradually increased from up to down of esophagus and extended from the inner muscular layer to outer muscular layer of esophagus.Conclusions:Unlike humans,the esophagus muscular layer of adult dog is mainly made up of skeletal muscle.The muscular layer of the whole esophagus in dog expresses the MHC-Ⅱ and MHC-Ⅰ,the expression of a-SMA mainly locates the muscular layer of lower esophagus.