Background: Muscle spindles are spe-cialized sensory receptors in the skele-tal muscle that respond to changes in muscle length and tension. Aim of the work: The aim of the present work is to compare the postnatal developmen-tal changes in the rat muscle spindles of masseter versus gastrocnemius. Ma-terial and Methods: 30 albino rats were used in this study. They were divided into 5 postnatal age groups, six animals in each group: newborn, 3 days, 6 days, 12 days and adults. Mus-cles were processed, and stained by Haematoxylin and Eosin (Hx&E), and Van Gieson's stains. Morphometric measurements were done and statisti-cally analyzed for the studied groups. Distribution maps of location of spin-dles were drawn. Results: The spindle contains 2 intrafusal fibers at birth. This number is increased with age till reached 4-6 fibers at age of 6 days then does not change up to adult age. Both nuclear bag and nuclear chain fibers are seen at all ages except at newborn where nuclear bag fibers are present alone. At birth, a thin incom-plete connective tissue capsule sur-rounds the spindle and is composed of 1-2 layers, located only in equatorial zone of spindles. With advance of age, capsule is increased in thickness, be-comes multilayered and extends to-wards polar regions of the spindles. A narrow capsular space is seen in the equatorial region of the newborn and becomes wider with older ages. Spin-dle morphometric values of masseter are significantly higher than that of gastrocnemius. Spindle maps show single spindles or as units located in deep part of both muscles near their nerve supply. Statistically significant increase with age advance is found in the spindle length, equatorial diameter of spindles, spindle area, and the whole muscle area. Notably, the num-ber of spindles is not changed, and the ratio of spindle area to muscle area decreases with age. Conclusion: There are statistically significant differences between the spindles in masseter mus-cle that is supplied by a cranial nerve and gastrocnemius muscle that is sup-plied by a spinal nerve. These differ-ences may be explained by the diverse functional roles of these muscles and their different embryological origins.