A significant effect of prenatal exposure to the pesticide Mirex is a fetal edema, which is responsible for many of the toxic effects of this agent. Pregnant rats were given 6 mg/kg mirex on d5½‐1½, a moderate dose that does not cause maternal mortality. The blood of 18½‐ and 20½‐d rat fetuses was tested to determine a possible cause of the edema. There were no significant differences between fetal and maternal plasma or between control and treated fetal plasma with respect to sodium and potassium concentrations or total osmolality. The protein concentration of plasma of treated fetuses was lower than that of controls. This effect was proportional to the degree of edema. The concentration in 20½‐d controls was 25.2 mg/ml, that in mildly swollen treated fetuses 18.1 mg/ml, and that in severely swollen ones 13.5 mg/ml. Polyacryl‐amide gel electrophoresis showed quantitative and qualitative changes in plasma of affected fetuses. The colloid osmotic pressure was also reduced from 8.0 mm in controls and unaffected treated fetuses to 4.8 mm in swollen fetuses. Since lowered plasma protein and colloid osmotic pressure cause tissue edema in adults, they are the probable cause of mirex‐induced fetal edema.