Background: Being a highly immature organ at birth, the rat kidney is morphologically as fetal when compared with humans. Events that occur during fetal development might determine adult renal diseases. Aim of work: This study aimed to characterize the postnatal developmental changes of the kidneys in albino rats using light microscope from postnatal day (PND) 2 until PND 70. This may give references to pathologists when evaluating juvenile toxicology studies. Materials and Methods: Ten healthy pregnant albino rats were used in this study. Twenty-five of their offsprings were obtained and divided according to age into five groups of 5 pups each. Group A: studied at PND 2. Group B: studied at PND 10. Group C: studied at PND 20. Group D: studied at PND 30. Group E: studied at PND 70. Kidneys were removed, processed for light microscopic study and 5 m thick paraffin sections were obtained and stained with hematoxylin and eosin stain. Results: Light microscopic examination of the renal cortex at PND 2 revealed subcapsular nephrogenic zone contained immature renal developmental stages, juxtamedullary zone contained formed glomeruli with medullary rays between the two zones. The renal cortex acquired maturation centrifugally with the superficial nephrons was the last to mature by PND 20. The papilla was the most mature region of the kidney and at PND2; it had the structural composition of the inner stripe of outer medulla. The papillary maturation involved a process of tubular elongation and increase in the interstitium until reaching adult structure by PND 20. At PND 2, the medulla was the most immature zone being formed of islets of tubular structures among abundant interstitium with high degree of undifferentiation. Its maturation involved tubular elongation and decrease of the interstitium with the outer medulla was the last to mature as late as PND 30. Consequently, the medulla remained immature for a relatively long postnatal period, in comparison to the other kidney regions. Conclusion: It was concluded that, rat kidney is immature at birth and kidney sub regions mature at different rates during postnatal development. The papilla was the first to mature (PND 20) followed by the cortex (PND 20) and finally the medulla (PND 30).