A number of investigators have shown that 5-hydroxytryptamine (5-HT) administration to pregnant animals produces changes in the placenta and leads to death of the fetuses. It has been well known that 5-HT is the most potent umbilical-placental vasoconstrictor and that its administration affects the nutritional function of the placenta. In view of these results it seemed desirable to determine 5-HT concentration in the placenta in experimental intrauterine growth retardation. IUGR was induced on the 17th day of gestation in pregnant rats by ligating the uterine artery of one horn as described by Wigglesworth with the opposite horn left untouched (control). On day 22, fetuses were delivered by C-section. 5-HT was determined by a fluorometric method. Statistical analysis employed the paired Student t-test. Average weight of IUGR fetuses was 3,05 g, whereas control fetuses 4,48. The mean concentration of 5-HT was 11% higher in IUGR fetal placentas. The mean placental concentration in IUGR was 247 micrograms/g of tissue, whereas in control the mean concentration was 222 micrograms/g of tissue. The difference between these placental concentrations was significant (p less than 0,01). We conclude that decreased blood supply to the pregnant rat uterus results in increased concentration in the placenta of 5-HT.