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The effects of location within the left or right uterine horn, position within each uterine horn, and fetal sex on fetal bodymass, blood flow to individual uterine segments associated with fetuses, and blood flow to the maternal portion of the placenta were investigated in rats. Sprague-Dawley rats were anaesthetized on day 5, 10, 15, 20, 21 or 22 of pregnancy, and radioactive microspheres with diameters of 15 mm were injected via a left ventricular cannula to measure blood flow to tissues. Tissues were weighed wet, and the rate of blood flow, corrected for wet mass (ml min-1 g-1 tissue), was calculated. Microspheres were not detected in fetuses, suggesting that they did not pass from maternal into fetal blood. Uterine blood flow was greater at the cervical and ovarian ends than in the middle of the uterus; on day 15 the rate of blood flow at the cervical and ovarian ends of each uterine horn was over twice that in the middle. The blood flow to the right uterine horn was greater than to the left horn. Blood flow to placentae increased dramatically between day 15 and day 20. There were marked differences in architecture between the uterine artery feeding the ovarian end of the right and left uterine horn, and blood flow to placentae located at the ovarian end of the right uterine horn was greater than to placentae in the same location in the left uterine horn. The blood flow to placentae and fetal bodymass were greater for female than for male fetuses on day 20, but on day 22 the reverse was observed.(ABSTRACT TRUNCATED AT 250 WORDS)