The extent of lipid peroxidation and antioxidation during pregnancy was studied in mothers and fetuses by treating pregnant spontaneous hypertensive rats (SHR) with 1.0%, 1.5% and 2.0% salt water and tap water (control). The treatment with 1% salt water was uneventful, and 2% salt water caused a marked increased in water consumption and occasional deaths in mothers. Study results for the 1.5% salt water (experimental) and control groups, unless otherwise specified were therefore compared at the end of the gestational period. 1) Systolic blood pressure was 170 mmHg before pregnancy in the control. The parameter dropped from day 17 of gestation down to 140 mmHg on day 20. In contrast, the parameter remained essentially constant throughout the gestational period in the experimental group. 2) The mothers in the experimental group had much higher triglyceride levels than the control. 3) Maternal serum and liver tissue concentrations of lipoperoxides were high in the experimental group. 4) Maternal liver tissue levels of antioxidant substances changed considerably during pregnancy; superoxide dismutase (SOD) activity and vitamin E were increased, but catalase activity was decreased. Glutathione peroxidase activity remained almost constant. There were no appreciable changes in lung tissue levels of antioxidant substances. 5) Placental tissues in the experimental group contained lipoperoxides and antioxidant substances in similar amounts to those in the control. 6) Fetal serum concentrations of lipoperoxides were much lower than the maternal levels in both experimental and control groups. In conclusion, placental tissues were believed to have a protective mechanism which prevents placental transfer of lipoperoxides from maternal to fetal blood.