Beef liver catalase (mol wt 240,000) was injected intravenously into normal rats and rats made nephrotic with aminonucleoside of puromycin. The localization of the tracer in the kidneys was then studied by ultrastructural cytochemistry, 3 min–12 hr after injection. Passage of catalase into the urinary space in normal rats was restricted by the basement membrane and by the epithelial slit pore. Nephrotic glomeruli showed extensive fusion of foot processes and formation of pockets and vacuoles in the fused epithelium; within 3 min after injection, catalase appeared in basal pockets, epithelial vacuoles, and the urinary space. Residual slit pores and close junctions in fused epithelium were impermeable to catalase. These studies indicate that alteration of the epithelial cells and basement membrane is responsible for protein leakage in aminonucleoside nephrosis.