Lactic acid dehydrogenase (LDH) levels in 100 normal pregnant women at various weeks gestation and 15 normal women during the 1st week of the puerperium were determined. The enzymatic activity was measured by following the optical density change at 1-min. intervals for 5 min. The mean LDH value for normal non-pregnant women was 19; a slight elevation of LDH was noted in 18% of the antepartum sera and in 100% of the postpartum patients.