572 This study was designed to test the hypothesis that regular training reduces the need for flow redistribution during exercise away from the splanchnic viscera in the mother and towards the brain in the fetus (induced by the fall fetal pO2). In order to test this hypothesis ultrasonic estimates of portal vein blood flow (PVF) were obtained in 5 control © and 6 regularly exercising (E) women before and in early (15±1 weeks), mid (26±1 weeks), and late (36±1 weeks) pregnancy. Measures of the velocity flow profile in the fetal middle cerebral (MCA) and renal (RA) arteries were obtained in another 16 C's and 20 E's at 30±1 weeks gestation. Duplicate measurements were obtained between 8 and 9 am, in the fasted state, in the upright position after 5 minutes of quiet standing and again immediately after 20 minutes of treadmill exercise at 55±3% of VO2max. A paired t-test and repeated measures ANOVA were used for data analysis. PVF at rest rose in both groups during pregnancy. Mean values were 505±70 cc/min prior to pregnancy and 805±135 cc/min and 765±130 cc/min in mid and late pregnancy. Post-exercise PVF's fell similar amounts in the two groups before pregnancy (71±6 vs. 60±11%) but, in mid and late pregnancy, the fall was greater in the C's (69±5 vs 48±9% and 59± 7 vs 30±9% respectively. In addition, post-exercise diastolic flow velocities were significantly increased in the fetal MCA and decreased in RA. These changes were significantly less in the fetuses of E's. We conclude that the hypothesis is correct and speculate that this benefits both mother and fetus during times of hemodynamic stress. Supported by NIH grants #HD21268, #RR0080 and The Fetal Diagnostic Center at MetroHealth Medical Center.