Maternal plasma progestagen concentrations increase about 20 days before parturition. The major contributors to the increase are reduced metabolites (ie 5α-pregnanes). Precocious increases (ie < 310 days of gestation) in these metabolites may occur in abnormal pregnancies. The effects of CRH, ACTH or betamethasone administered to the foetus at gestational ages ranging from about 250 to 320 days were examined. Sixteen healthy pony mares were used for foetal injection employing aseptic techniques. Water or normal saline were used as controls. Maternal plasma progestagen concentrations were measured using a commercial radioimmunoassay (RIA) progesterone kit and results were confirmed using gas chromatography-mass spectrometry (GC-MS). Results demonstrated clearly that an increase in maternal plasma progestagen concentrations occurred after injection of ACTH, CRH or betamethasone to the foetus, irrespective of gestational age. A comparable increase was not observed in the control animals. Of the 16 mares in which the foetus was injected, 13 produced viable foals at gestational ages ranging from 307 to 339 days whereas 3 mares delivered non-viable foals at 284 to 306 days gestation. The results support the hypothesis that the pre-parturient rise in progestagens occurring in the mare is the result of foetal adrenocortical activity.