Blood estrone concentrations of 65 pregnant dairy cows were measured by an Ittrich-Kober fluorometric procedure. Average estrone increased slowly from 1.2ng/ml at 16 to 14 weeks prepartum to 2.5ng/ml at 8 to 6 weeks prepartum, and then more rapidly to 4.8ng/ml at 4 to 2 weeks before parturition. The highest average, 8.3ng/ml, was 5 days preparturn; no sanlples were taken closer to parturition. The lowest average (0.7ng/ml) mas 5 days postpartum.