"Essential" phospholipids (EPL; polyene-phosphatidylcholine) were administered orally to aging male rats in doses of 100 and 300 mg/kg b.wt. over 10 weeks. One and 7 days after the last treatment cytochrome P-450 concentration, epoxide hydrolase and glutathione-S-transferases were found to be unchanged, but ethylmorphine N-demethylation, ethoxycoumarin O-deethylation and UDP-glucuronosyltransferase activities were enhanced, as were the concentrations of both reduced and oxidized glutathione, the values being similar to those in young adult rats. These results are discussed in terms of an increase in endoplasmic reticulum membrane fluidity.