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To evaluate the extent to which endogenous hormones in peripheral blood can account for uterine blood flow (UBF), rates of blood flow and concentrations of estrone (E1), estradiol (E2), and progesterone (P4) were measured simultaneously in four sheep during the estrous cycle and the first 3 wk of pregnancy. During the estrous cycle, UBF was inversely related to the concentration of P4 (P less than 0.01) and directly related to (E1 + E2)/P4 (P less than 0.01). There was no animal in which UBF was related to estrogen alone. No statistically significant relationship between endogenous hormones and UBF was detected in any ewe during the first 3 wk of pregnancy. Data support the idea that UBF varies during the estrous cycle in relation to peripheral concentrations of P4 and (E1 + E2)/P4 but leaves open the nature of the relationship in early pregnancy.

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