The present study aimed to throw more light on the effect of the insecticide monocrotophos on the kidneys of mice treated orally with the insecticide. The applied doses were ¹/₂₀, ¹/₆ and ¹/₃ LD₅₀ (0.38, 1.25 and 2.5 mg/kg body weight), respectively. Each dose was given for seven successive days. The observed histopathological and ultrastructural alterations were found to be dose-dependent. The low dose (¹/₂₀ LD₅₀) caused shrinkage of some glomeruli; some were congested and the cellularity in some others were increased. Both of the proximal and distal convoluted tubules cells showed many cytoplasmic vacuoles and pyknotic nuclei and disappearance of microvilli of the proximal convoluted tubule cells. The degenerative alterations became more severe in case of the median (¹/₆ LD₅₀) and the high (¹/₃ LD₅₀) doses of monocrotophos. In such cases, the glomeruli revealed cellular abnormalities, necrosis and some of them were deeply clefted. Dissociated and degenerated renal tubule cells with pyknotic nuclei, and cell debris in the tubules lumina were observed in the kidney cortex. In case of the cumulative treatment, monocrotophos in a dose of 0.19 mg/kg given for twenty successive day, caused similar degenerative changes to the afore-mentioned alterations. These histopathological alterations showed partial recovery when inspected on the seventh day after the last dose of treatment, and the recovery became more pronounced on the fourteenth days after the last treatment. This may indicate withdrawal of the insecticide from the experimental animal after stopping treatment. The ultrastructural alterations showed increased mesangial matrix and atrophied podocytes with fusion of their secondary foot processes. The cells of the convoluted renal tubules displayed degenerated mitochondria, short profiles of RER, pyknotic and chromatolytic nuclei and presence of cytosegrosomes (sequestration of parts of their cytoplasm).