Pharmacologic interruption of the angiotensin II (ANG II) type-1 receptor signaling during nephrogenesis in rats perturbs renal tubular development. This study aimed to further investigate tubular developmental defects in neonatal rats subjected to ANG II inhibition with enalapril. We evaluated tubular ultrastructural changes by electron microscopy and estimated spectrophotometrically activity or concentrations of succinate dehydrogenase (SDH), cytochromes a and c, which are components of mitochondrial respiratory chain, on postnatal day (p.d.) 2 and 9. Renal expression of sodium-potassium adenosinetriphosphatase (Na⁺-K⁺- ATPase) and two reflectors of mitochondrial biogenesis- mitochondrial transcription factor A (TFAM) and translocase of outer mitochondrial membrane 20 (TOM20)- was also studied using western immunoblotting and immunohistochemistry. Enalapril disrupted inner mitochondrial membranes of developing cortical and medullary tubular cells on p.d. 2 and 9. These findings were paralleled by impaired mitochondrial respiratory function, as revealed from the changes in components of the mitochondrial RC, such as decreased cytochrome c amount in the cortex and medulla on p.d. 2 and 9, decreased cytochrome a amount in the cortex and medulla on p.d. 2, and diminished cortical SDH activity on p.d. 2 and 9. Moreover, tubular expression of the most active energy-consuming pump Na⁺-K⁺-ATPase was decreased by enalapril treatment. Renal expression of TFAM and TOM20 was not altered by neonatal enalapril treatment. As nephrogenesis is a highly energy-demanding biologic process, with the energy being utilized for renal growth and transport activities, the structural-functional alterations of the mitochondria induced by neonatal enalapril treatment may provide the propensity for the tubular developmental defect.