RESUMEN
High ambient temperature (34.4 C degrees and 60-65% relative humidity (RH failed to maintain optimum embryonic and/or foetal survival rate in ovariectomized pregnant rats given exogenous progesterone and oestrone injections. A still higher ambient environment (36.6 C degrees and 70-75% RH) did not affect the decidual cell response (DCR) in psuedo-pregnant rats. Heat stress did not result either in insufficiency or lack of progesterone and hence the latter two are not factors for the heat stress causing embryonic and/or foetal loss. The possibility of the failure of oestrone being converted to oestradiol or an intrinsic interference of RNA synthesis to be the factors responsible for embryonic and/or foetal loss caused by heat stress has been examined.