The effect of di-2-ethylhexyl phthalate (DEHP) on lipid metabolism was studied in liver and brain from fetal rats taken 3 days before parturition from dams receiving dietary DEHP during gestation. In fetuses from rats receiving 0.5% or 1.0% DEHP in a stock diet, the incorporation of 14C-acetate and labeled mevalonate (3H or 14C) into the C27 sterols, C30 sterols, and squalene fractions of brain tissue incubated in vitro was significantly reduced between the confidence limits P <0.05 to P <0.001. When liver from fetuses was incubated with labeled mevalonate, incorporation of label into the C27 sterol and C30 sterol fractions was significantly reduced as well (P <0.02 to P <0.001), whereas incorporation of labeled mevalonate into the squalene fraction was not significantly altered. The incorporation of 14C-acetate into total hepatic lipids of the fetal rats was also studied, and statistically significant reductions in incorporation were observed in the lanosterol fraction (P <0.001), the combined fraction of sterol esters + squalene (P <0.02), and the combined fraction of cholesterol + diglycerides (P <0.01). No significant changes were observed in the incorporation of 14C-acetate into phospholipids, free fatty acids, or triglycerides. In 8-day old suckling rats delivered from dams fed 0.5% DEHP for the last 16 days of gestation and maintained on the same diet during the nursing period, the incorporation of 14C-mevalonate into hepatic C27 sterols, in vitro, was significantly depressed (P <0.05) whereas in corporation into C30 sterols and squalene was similar to control values. In these same suckling rats, body weights were significantly lower in the control group (21.7 vs. 18.8 g, P <0.01), whereas liver weight as a % of body weight was significantly higher (P <0.01) in rats nursing from the DEHP-fed dams. The results ind'teate that the inhibitory effect of dietary DEHP on lipid metabolism in the mature rat is transmitted across the placental barrier to the developing fetus and that the abnormal pattern of lipid metabolism in rats delivered from DEHP-fed females is only partially restored to normal during the suckling periods.