The present study was undertaken in order to determine the changes in blood levels of lipid peroxide and vitamin E during pregnancy. 1. The man plasma level of lipid peroxide was 0.89 +/- 0.13 nmole/ml for non-pregnant women, while in pregnant women it was increased as the progressed pregnancy reaching 2.71 +/- 0.29 nmole/ml at full term. 2. The mean plasma, red cell and platelet levels of vitamin E were 6.6 +/- 0.9 micrograms/ml, 3.55 +/- 0.38 micrograms/ml packed cell and 99 +/- 25 micrograms/g protein for non-pregnant women, while in pregnant women the levels changed as the progressed pregnancy becoming 15.5 +/- 0.9 micrograms/ml, 2.43 +/- 0.24 micrograms/ml packed cell, and 244 +/- 19 micrograms/g protein at full term. It was shown in this study that during pregnancy the plasma level of lipid peroxide increased while the levels of vitamin E which inhibited the formation of lipid peroxide, increased in plasma and platelets, but decreased in red cells.