Of 48 spare human pre-embryos achieving the expanded blastocyst stage, 22 (45.6%) secreted significant amounts of human chorionic gonadotrophin (HCG) (> 5 IU/l/day). Of these, nine remained intrazonal, seven partially hatched and six fully hatched. Embryonic production of HCG in vitro appeared to be time-dependent, starting after a certain minimum time (approximately 160 h post-insemination) and rising exponentially, with maximal HCG production around day 10. Hatching was not a prerequisite for HCG secretion, since similar amounts were produced by intrazonal blastocysts. Blastocysts derived from abnormally fertilized oocytes also began secreting HCG exponentially but secretion was delayed and the upper limit of maximum HCG secretion rate was comparatively low. The actual amount of HCG is thought to reflect the number of viable trophectoderm cells producing the hormone. HCG doubling times for blastocysts in vitro were rapid when compared to implanting blastocysts of a similar age in vivo, with 19/22 (86.4%) blastocysts having a doubling time of < 10 h. Provided a pre-embryo can secrete HCG and maintain an adequate doubling time, sufficient HCG should be produced for initial stages of embryonic recognition in vivo. Since intrazonal blastocysts are capable of fulfilling both of these criteria, the limiting factor in realizing their full potential may be escaping from the zona pellucida.