Most modern highly productive poultry strains have been developed in temperate countries, with little opportunity for heat tolerance to be a selection factor. When these birds are moved to tropical or subtropical countries their egg production, egg quality, food intake and growth rate decrease at high ambient temperature. Varying reports on the effects of heat stress on egg and yolk production may be attributable to the differences of the range and duration of thermal loads employed, different bird strains, ages, diets and food intake responses. There are, however, only a few studies which attempt to explain the actual physiological mechanisms involved in the adaptation of laying hens to thermal loads, especially those relating to the changes in egg yolk precursors including vitellogenin and very low density lipoprotein (VLDL) synthesis, secretion and deposition in the oocytes. The major yolk precursors are vitellogenin, a metal binding protein, and VLDL, a specialised type of triglyceride-rich lipoprotein found in abundance in the plasma of laying hens. Both vitellogenin and VLDL are synthesised in the liver in response to oestrogen stimulation transported to the ovary and transferred and deposited into the growing oocytes by a selective mechanism. Chronic heat stress in laying hens reduces yolk size and total yolk production considerably. Any changes in yolk precursor synthesis mediating such a response may involve altered oestradiol secretion or reduced sensitivity of hepatocytes to oestrogen stimulation. Other possible mechanisms of reduced yolk accumulation include disturbances in the transport of vitellogenin in the blood stream and changes in the level of ovarian uptake of egg yolk precursors by the growing oocytes. The present project examined the changes in egg yolk precursor levels in the plasma of laying hens chronically exposed to different degrees of thermal stress (32 and 35°C with various relative humidities) and precursor uptake by the oocytes. The study also addressed the role of oestrogen in yolk precursor responses to chronic heat stress. In addition, the role of nutritional vitamin E supplementation as a strategy for alleviating the effects of heat stress has been examined. The studies have confirmed that heat stress reduces yolk and egg production concomitant with reduced circulating vitellogenin and VLDL in the plasma.