Background: The functional and morphological development of the glomerlus is an essential part of nephrogenesis that takes place during Intra uterine life and early post-natal days. Objectives: To study the morphometric parameters of the maturation of the glomerular filtration slit of the kidney during neonatal period and its implication on the function of the kidney. Method: Rat litters are used as a mammalian model of the development during the first 8 days of post -natal life. Image analysis using Bell software system was used for morphmetric measurement of filtration silt as examined by electron microscope. Results: The width of the filtration slit of new born kidney was 11.24±1.6 nm which increased by increment to be 14.83±1.6 nm on 4th.post- natal day. Eventually reached 25.34±4.3 nm on day 8 post natally. Conclusions: In New Born litters, the glomerular filtration slit is narrower than that of mature kidney. Glomerular development continues to take place during first post natal week to reach the adult mature parameters at the end of neonatal period. This explains the low GFR found in new born mammals.