A b s t r a c tElevated body temperature can result from many agents in the natural environment, such as fever, hot weather and heavy exercise. In our modern living conditions additional sources of induced hyperthermia including hot baths, saunas, drugs, electromagnetic radiation and ultrasound have been introduced. Hyperthermia during pregnancy has been shown to cause a wide spectrun of effects in all species studied, including humans, the outcome depending on the dose of heat absorbed by the mother and embryo and the stage of enbryonic or fetal development when exposed. The dose of heat is the product of the elevation of temperature above normal and the duration of the elevation. In relatively uncontrolled natural environmental exposures, embryonic death and resorption or abortion are probably the most common outcome. In less severe exposures (smaller doses) major or minor developmental defects can result and the central nervous system appears to be a major target for its effects. Heat damage to embryos appears to be by apoptotic and other forms of cell death in organs at critical stages of development. Over many millennia all living orgaisms appear to have developed protective m e c h a n i s m s against excess heat, known collectively as the heat shock response. This response has been s t u d i e d intensively over the last 20 years and its mechanisms of protection are now becoming more clearly defined. Exposures to heat (and a number of other toxic agents) trigger the heat shock response which is characterized by abrupt suspension in the normal protein synthesis and the concurrent induction of heat shock genes (hsp) and the synthesis of a set of protein families known collectively as the heat shock proteins (HSP). The hsp are known to be involved in the response in embryos, each has at least two copies, one which appears to have functions in the normal embryonic development (cognate) and another which is induced at a certain dose of heat (induced) and which can offer some protection against damage for some time after the initiating dose.