The relationship between regional variations in uterine blood flow and fetal-placental weight/size was examined in guinea pigs possessing three pregnancy sites per horn. Pregnancy sites were localized in either the tubal, middle, or cervical third of the uterus. Throughout pregnancy, the middle uterine pregnancy sites consistently received a lower blood flow rate than did either of the other two uterine zones. The middle zone also possessed the lowest fetal-placental weights and smallest fetuses, based on crown-rump length measurements, of all three zones. Significant correlation coefficients were found to exist between uterine blood flow and placental weight, placental and fetal weights, and fetal weight and crown-rump length. Experimental reduction of uterine blood flow to the tubal pregnancy site effectively induced a retardation of intrauterine fetal-placental growth, which was comparable to that found in the middle uterine zones. These data indicate that variations in regional blood flow rates to the guinea pig uterus affect intrauterine growth of the fetal-placental unit.