The goal of this paper is to describe the prenatal developmental processes of the thermoregulatory system occurring during the last third of embryogenesis in precocial embryos and its influence on the postnatal efficiency of thermoregulatory control elements. For this, in embryos of the domestic chicken (Gallus gallus) and the Muscovy duck (Cairina moschata) the internal core temperature, the heat production and oxygen consumption, respiration rate after internal pipping, the peripheral blood flow in the chorioallantoic membrane were measured and the Q(10) of heat production was calculated. The experiments show that (1) thermoregulatory behaviour is developed after internal pipping in avian embryos, (2) avian embryos show endothermic reactions, (3) hyperthermia causes a down-regulation of heat production in precocial avian embryos, (4) in contrast to heat production, the efficiency of heat loss is high in precocial embryos, (5) the development of physiological control systems starts with non-coordinated and proximate (immediate) non-adaptive reactions. It is concluded, that strategies of avian embryos in relation to temperature regulation are developed optimally. Endothermic reactions occur very early during the embryonic development but their efficiency is limited. Due to the van't Hoff rule, low temperatures decrease the net heat production but the trajectories of endothermy are stimulated and related epigenetic adaptation mechanisms are activated. Using behavioural mechanisms, the embryo is protected against super-cooling during the natural incubation process. The heat loss mechanisms are most efficient against heat stress, which occurs only for a very short time during incubation and protect the embryo against disturbances caused by hyperthermia. (C) 2004 Elsevier B.V. All rights reserved.