Diudecyl phthalate (DUF) was fed to groups of five male and five female Fischer 344 rats at dietary levels of 0(control) 0.3, 1.2 or 2.5% for 21 days. A further group of five rats of each sex, fed 1.2% DEHP served as a study control. Bodyweight gain was reduced in both sexes fed 1.2 and 2.5% DUP. Absolute liver and kidney weights were increased in these groups, as was the activity of Palmitoyl-CoA oxidase. Serum triglyceride and cholesterol levels were reduced in the male 1.2 and 2.5% DUP groups, there was no effect in the females. There was an increase of 11- and 12-hydroxylase activity in all three DUP dose groups in the males and in the top dose group (2.5 DUP) in the females. Electron microscopic examination of liver sections from two rats of each sex from the 2.5% DUP group showed a moderate increase in peroxisome number and size.