The radioimmunoassay technique was used to measure maternal, placental and fetal serum gastrin levels. A progressive increase in gastrin levels occurred with the advance of pregnancy reaching mean peak levels of 62 (SEM 12) pg/ml during labour. Umbilical artery (74 pg/ml: SEM 7) or vein (73 pg/ml: SEM 7) gastrin concentration was higher than maternal gastrin levels. Placental gastrin concentration was 840 pg/g (SEM 96) and amniotic fluid contained 22 pg/ml (SEM 2). This study implies that one of the reasons for increased maternal gastric residue seen during pregnancy is due to an increased serum gastrin concentration. Also the advanced stage of development of gastrointestinal system of the newborn is in accord with high fetal gastrin concentration.