The purpose of the study is to note the effects of the resection of the lateral pterygoid muscle on the mandibular growth in the growing rats. Twenty four female Wistar rats were used in the experiment. They were divided into three groups´ group 1, bilateral sham operation group 2, bilateral resection of the lateral pterygoid muscle group 3, unilateral resection of the lateral pterygoid muscle (The right lateral pterygoid muscle was resected and the left one was sham-operated) Groups 1&2 were sacrificed eight weeks later and group 3, four weeks later. All specimens were measured with calipers, and stained with hematoxylin and eosin. Groups 1&2 were compared with each other. In group 3, the right side was compared with the left. The results were as follows 1. In the growing rats, the resection of the lateral pterygoid muscle was followed by a decrease of the mandibular growth. After eight weeks, the condyle-mental foramen distance, the angular process-mental foramen distance, the size of the condylar head, the supradentale-first molar distance, and the diameter of the symphysis were significantly smaller than the control. 2. Resection of the lateral pterygoid muscle resulted in decrease of the thickness of the cartilage layer and the prechondroblastic-chondroblastic layer after four weeks. 3. After eight weeks, group 1 and group 2 were not different significantly in the histologic sections. 4. The condylar cartilage was stabilized eight weeks after the experiment.