The effects of maternal ingestion of two cups of coffee were investigated in 20 pregnancies during the last trimester. Maternal serum caffeine and epinephrine concentrations after 30 minutes were significantly elevated as compared with the fasting values (p less than 0.01). The intervillous placental blood flow decreased almost significantly (p less than 0.05). The fetal umbilical vein blood flow was unchanged. In patients with hypertensive pregnancy in the series there was reduced intervillous blood flow initially, and these values did not change after the maternal caffeine intake. The decrease of placental blood supply and increased maternal serum epinephrine levels associated with maternal coffee ingestion may be potential perinatologic risks, and more investigation about caffeine effects in human pregnancy is needed.