Morphological effects on the medial pterygoid muscle were evaluated in 20 male gerbils (average weight, 55 g) after occlusal alterations induced by extraction of left side molar teeth. Controls were only submitted to surgical stress. Sixty days after surgery, the groups were divided into two subgroups for the following studies: (1) observation of macroscopic morphology and vessels distribution (n=10); (2) light microscopy histological analysis (n=10). Group results were statistically compared using the Wilcoxon and Mann-Whitney tests, with a significant value of p<0.05. Statistical differences in biometric data were found between the left and right sides of the experimental group (p=0.043), and between the left side of the control group when compared to the same side in the experimental group (p=0.044). Vessels supplied by bundles of the external carotid artery in the medial pterygoid muscle did not show distribution differences in group comparisons. Histological alterations were found in the ipsilateral side of the experimental group, with a central localization of the nucleus and degenerative aspect of the fibers, usually near to internal aponeurosis. Fiber diameters seemed reduced and the neuromuscular spindles were localized near internal aponeurosis and had a modified appearance. It is concluded that the medial pterygoid muscle in the gerbil is sensitive to alterations of the masticatory movements.