The endocrinology of ectopic pregnancy was studied in order to investigate the origin of the discordance in the circulating amounts of human chorionic gonadotrophin (HCG) and those of oestradiol and progesterone. Serial maternal blood samples were obtained at 4-9 weeks gestation from 93 patients who became pregnant following in-vitro fertilization and embryo transfer including 10 ectopic, 21 anembryonic and 62 normal singleton pregnancies. The samples were analysed for HCG, Schwangerschaft protein-1 (SP-1), pregnancy-associated plasma protein-A (PAPP-A), progesterone and oestradiol. In ectopic pregnancies, concentrations of all substances analysed were significantly reduced compared to singleton pregnancies from 5 weeks gestation (P < 0.05-0.001) but they were not significantly different from those of anembryonic pregnancies. In ectopic pregnancies, associations were found between the concentration of both HCG and SP-1 and those of progesterone and oestradiol. No associations were found between PAPP-A and any other substances analysed. This may be due to insensitivity of the PAPP-A assay; alternatively PAPP-A concentrations may be differentially reduced in ectopic pregnancy. These findings suggest that progesterone and oestradiol are derived from the corpus luteum in early ectopic pregnancy but that the corpus luteum fails rapidly and the dominant source of both hormones becomes the trophoblast as early as 5 weeks.