DNA polymerase activity has been measured in placentas of normal and protein-restricted rats and correlated with the mean percent daily increase in DNA. During normal placental growth, increases in DNA fell rapidly from 13 to 19 days and polymerase activity using denatured DNA template showed a similar pattern falling from values of 10,000 mumu mols dAMP incorporated per mg DNA at 12 days of gestation to 3,100 at 19 days. Protein restriction during gestation reduced placental DNA content after 14 days; by 19 days the DNA content was 81% of normal. The increase in DNA between 13 and 19 days in placentas of malnourished animals paralleled the normal but was significantly lower. Malnutrition markedly reduced enzyme activity at 12, 14, and 16 days; at 19 days, when DNA synthesis has normally ceased, values of DNA polymerase were not different in control and malnourished placentas. Thus DNA polymerase activity using denatured DNA as template, as measured in vitro, was an index of proliferative cell growth in both normal and malnourished placentas. Furthermore, the decrease in enzyme activity in malnourished samples preceded by at least two days any measurable decrease in total placenta DNA content. It is suggested that future clinical application of this technique may provide an index of nutritional status in "at risk" pregnancies.