Blood flow to each uterine horn of cows during the oestrous cycle and early pregnancy was determined daily by use of electromagnetic blood flow probes placed around both middle uterine arteries. The pattern of blood flow to uteri of pregnant and non-pregnant cows was similar until Day 14 after mating or oestrus. Between Days 14 and 18 of pregnancy blood flow to the uterine horn containing the conceptus increased (P less than 0.01) 2- to 3-fold, whereas blood flow to the non-gravid uterine horn in these cows remained constant. No corresponding increase in blood flow to the uterine horn ipsilateral to the ovary bearing the CL was observed in non-pregnant cows during this 4-day period. By Day 19 of pregnancy, blood flow to the gravid uterine horn had returned to a level similar to that observed on Day 13. Blood flow to both uterine horns of pregnant cows remained constant from Days 19 to 25 and then increased to the gravid horn (P less than 0.01) markedly until Day 30 whereas blood flow to the non-gravid horn remained low. Uterine blood flow during the oestrous cycle of non-pregnant cows was positively correlated (P less than 0.01) with systemic concentrations of oestradiol and the ratio of oestradiol (pg/ml) to progesterone (ng/ml). There was no association between oestradiol concentrations and blood flow to the gravid uterine horn. These data indicate local control of uterine blood flow by the bovine conceptus which may function to create optimal conditions for the continuation of pregnancy.