The intestinal permeability of aging rats to various molecular weight species of polyethylene glycol 400 (PEG 400) was studied. Animals received a bolus of PEG 400 by oral gavage and urine was collected for 6 h to assess its rate of absorption. Quantitation by gas liquid chromatography revealed that total urinary excretion of PEG 400 increased with aging. 34-week-old rats excreted 34.3% of the administrated dose while 43.6% was excreted in the urine by rats 133 weeks of age. This effect was more pronounced with the higher molecular weight PEG since excretion of the lower molecular weight PEG 282 decreased by 10.5% while PEG 634 excretion increased by 11.4% with aging. The increased permeability of the intestinal tract to higher molecular weight species of PEG may indicate that the intestinal protective barrier to the absorption of potentially harmful environmental substances may be less efficient in aging animals. If similar findings are found in aging humans, they may indicate increased potential for absorption of large antigenic or carcinogenic compounds from the intestinal lumen.