The effect of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) on acetyl-CoA carboxylase (ACC) activity and synthesis was examined. Male Wistar rats received a single i.p. injection of TCDD (53 micrograms/kg), and nine days later body weight, liver weight, hepatic lipid, ACC activity and mass were determined and compared to pair-fed controls. Body weights of TCDD-treated animals decreased, while liver weights increased resulting in an increase in liver to body weight ratios. ACC activity was decreased by 65%, however sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) and Western analysis using a biotin specific probe revealed that ACC protein levels were not appreciably changed. In addition, there was a large increase in exogenous lipid material in TCDD-treated livers as determined by osmium tetroxide staining. These data suggest that the decrease in ACC activity may be due to direct inhibition of the enzyme by negative allosteric interactions with free fatty acids released from adipose tissue that subsequently accumulate in liver.