The effects of experimental intrauterine growth retardation on fetal and placental weight and placental blood flow were studied in rats on day 21.5 of pregnancy. Two experimental approaches were employed. (1) Uterine artery ligation on one side on day 18.5 induced large individual variations in fetal and placental weight as well as in placental blood flow. We showed that fetal weight and placental blood flow were closely related for the values of flow ranging from 0.2 to 1.0 ml.min-1 and the values of fetal weight ranging from 2.5 to 4.5 g. In the opposite horn (control horn), variations of flow ranged from 1.0 to 2.2 ml.min-1 and they occurred without notable change of fetal weight. A relationship between fetal and placental weight was observed as well. In the ligated horn smaller fetuses had smaller placental weight. (2) Maternal fasting, 48 hours before term i.e. between 19 and 21 day of pregnancy decreases both fetal weight and placental blood flow by about 10% and 50% respectively as compared to the control values on day 21. Placental weight is virtually unchanged. The present data allow us to conclude that a marked decrease in maternal placental blood flow might be one of the main causes leading to intrauterine growth retardation.