Peripheral blood plasma was analyzed for total estrogen and progesterone concentrations concurrently during several reproductive stages of the cow. In trial I, 18 beef heifers were mated and blood was collected every 3 days until they either returned to estrus or until day 39 of pregnancy. In trial II, 10 dairy cows were bled daily from 7–14 days prior to parturition, then on a twice a week basis for 60 days postpartum. Estrogen and progesterone concentrations were determined by radio assay. In mated cows that returned to estrus, the mean estrogen level was below 7 pg/ml on day 3, 6, 12 and 15. It increased to a minor peak on day 9 (13 pg/ml) and to a major peak just before estrus (25 pg/ ml). In pregnant cows, it was low (Endocrinology90: 1336, 1972)