The patterns of urinary proteins in rats of different ages were examined on SDS gradient polyacrylamide gel electrophoresis coupled with silver staining. Proteins were fractionated into at least 26 bands. Densitometric measurements were used to characterize protein excretion patterns. The results showed that proteinuria in newborn, young and adult rats is predominantly tubular, consisting of low molecular-weight species. Conversely, late adults and old rats had a mixed glomerular pattern, with a steadily increasing excretion of albumin, IgG and transferrin, as was the case of other high molecular-weight proteins. Fragments of both immunoglobulins and albumin were found in all urine samples assayed. In 1 month old rats the percentage of Tamm-Hörsfall (T-H) protein was higher (P < 0.01) than in the remaining groups studied. In newborns, relatively high albumin, IgG and transferrin percentages were detected, as well as an alpha 1-acid glycoprotein and carbonic anhydrase excretion (P < 0.05 and P < 0.01 respectively) higher than that observed in the other age groups studied.