Indomethacin (1, 10 and 100 mumol L-1) was incrementally infused (8 min for each concentration) into the maternal and fetal circulations of dual-perfused human placental lobules, and measurements were made of materno-fetal amino acid transfer (using the non-metabolizable amino acid alpha-aminoisobutyric acid, or AIB) and fetal effluent perfusate prostaglandin (PG) E levels. No significant changes in AIB transfer compared with controls were observed at any of the doses of indomethacin infused. There was, however, a significant dose-dependent reduction compared with controls in PGE in the fetal circulation with the infusion of indomethacin.