To study aspects of protein homeostasis and proteinuria, experiments were done on normal rats and on rats 1–6 days after a single intravenous injection of the aminonucleoside of puromycin. Albumin excretion of normal rats, measured by a sensitive radioimmunoassay, was 0.139 mg/24 h. Following the injection of aminonucleoside, albumin excretion first increased on the second or third day, and increased thereafter exponentially with time. In 0.5 µm frozen substituted sections of kidney tissue there were a few droplets of albumin, but not of γ-globulin, in the proximal tubules of control rats. There was little change one day after aminonucleoside. Large numbers of protein reabsorption droplets, containing albumin and γ-globulin, were seen in both proximal and distal tubular cells of rats 4–6 days after aminonucleoside. This finding is consistent with increased reabsorption from the lumen by both proximal and distal tubules and, inferentially, with increased glomerular permeability to both proteins. Six days after aminonucleoside, two populations of tubules were observed; one relatively normal in appearance and containing an increased number of droplets; the second dilated, and containing massive amounts of both proteins in lumen and cells. This observation suggests some heterogeneity of nephron damage and function following aminonucleoside.