The purpose of this study was to analyse the postnatal development of the intrinsic laryngeal muscle fibers using the myosin ATPase stain method . We measured the ratio of type II c fibers in all of the intrinsic laryngeal muscle fibers in postnatal-day (PND) 1 , 3, 7, 14, 21 and mature Sprague-Dawley rats . Type II c fibers disappear at PND 14 in most intrinsic laryngeal muscle fibers . However, the decreasing rate of the fibers is different for various intrinsic laryngeal muscles . Decreases

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... n type II c fibers are observed most in the lateral compartment of the thyroarytenoid muscle (1-TA), and then in the posterior cricoarytenoid muscle (PCA) , cricothyroid muscle (CT), lateral cricoarytenoid muscle (LCA), interarytenoid muscle (IA) and the medial compartment of the thyroarytenoid muscle (m-TA). These results suggest that the differentiation of the intrinsic laryngeal muscle fibers in rats is nearly accomplished by the end of the second week .