Renal cyclooxygenase-2 (COX-2) expression is necessary for normal glomerular development. Impaired COX-2 activity has been associated with decreased expression of VEGF, a vascular growth factor of significant importance for glomerular capillary development. Therefore experiments were designed to test the hypothesis that COX-2 expression supports glomerular development through stimulation of capillary loop formation. A developmental series of COX-2 knock-out (KO) mice and wild-type littermates (WT) was used. Animals were sacrificed by decapitation (P7, P14) or after anaesthesia using a mixture of ketamine (50 mg/kg) and xylazine (10 mg/kg) and kidney tissue snap frozen in liquid nitrogen or fixed in 4% paraformaldehyde. The number of glomeruli was counted by stereology, and glomerular morphology was examined by electron microscopy. mRNA expression of COX-2 and vascular growth factors were examined by quantitative PCR. Renal COX-2 expression showed developmental regulation with significantly higher COX-2 expression at postnatal (P) days 1 and P7 than at any later time points (P14, P21 and P40). Quantitative unbiased stereology at P28 showed significantly reduced glomerular number in COX-2 KO mice compared to WT (8188 ± 781 and 12251 ± 454 glomeruli/kidney respectively, P=0.0001). Subcapsular accumulation of small and immature glomeruli was seen in COX-2 KO mice whereas glomeruli deeper in the cortex towards the medullary junction appeared normal. Tissue abundance of VEGF, angiopoietin-1 and -2 mRNAs was significantly reduced in COX-2 KO mice compared to WT at P7. By electron microscopy at P28, immature subcapsular glomeruli showed normal appearance of fenestrated endothelial cells but severe podocyte foot process effacement and absence of mesangial cells. Normal morphology was confirmed in glomeruli deeper in the cortex. At P7, COX-2 KO mice showed reduced expression of slit diaphragm proteins nephrin and podocin but not synaptopodin compared to WT. In summary, deletion of COX-2 leads to decreased renal expression of vascular growth factors without changes to glomerular endothelial cell ultrastructure and severe podocyte foot process effacement and impaired expression of slit diaphragm proteins. In conclusion, COX-2 activity is necessary for normal podocyte function and slit diaphragm formation during late stages of kidney development.