Male Wistar rats received an intraperitoneally daily dose of thioacetamide of 100 mg/kg body weight during 3, 8 and 30 days. Phospholipid levels and radioactive incorporation of 3H2O into total lipids, polar lipids and phospholipids 1 h after intraperitoneal administration of 5 mCi of tritiated water, were determined in livers of untreated control rats and of animal treated during the above mentioned periods. Earlier doses of thioacetamide provoked an increase in the 3H2O incorporation into total lipid fraction. Prolonged treatment resulted in an inhibition of tritiated water incorporation. After three days of thioacetamide administration the incorporated radioactivity in phospholipid fraction decreased. There was also observed a decrease in the radioactivity incorporated into the major phospholipids: phosphatidylcholine and phosphatidyl ethanolamine. However, the levels of these phospholipids showed no significant changes. These observations support the suggestion that de novo biosynthesized fatty acids are being utilized for 1-palmitoyl-2-oleoyl and 1-palmitoyl-2-linoleoyl phospholipids biosinthesis in lesser proportion in rats treated with thioacetamide than in control rats.