The intraperitoneal injection of 1 g of bovine serum albumin daily for 5 days was shown by electron-microscope morphometry to cause swelling of the glomerular epithelial cells and very severe loss of foot processes. However, these changes were found in only 70 per cent. of glomeruli and the other 30 per cent. remained normal. After 7 days' recovery following five daily injections of 1 g of bovine serum albumin, the swelling of the glomerular epithelial cells had subsided and the foot process reappeared. These changes were accompanied by severe proteinuria which resolved only slowly when the injections were stopped. After daily injections of 0-8 g of egg albumin for 5 days there was no swelling of the glomerular epithelial cells and only very slight loss of foot processes detectable only by morphometry. There was a less severe proteinuria than after injections of bovine serum albumin and it resolved more rapidly when injections were stopped. It is suggested that these differences arise from the fact that bovine serum albumin is reabsorbed by the glomerular epithelial cell but egg albumin is not. Two of four rats allowed to recover for 7 days after five daily injections of 1 g of bovine serum albumin had unusual glomerular lesions.