Broiler chickens 4 to 8 weeks old and turkeys 10 to 19 weeks old, respond similarly in their performance, to an increase in heat load. In chickens the response function of body weight to an increase in relative humidity (rh) was bell-shaped with a maximum at 60-65%, when T_a ranged from 28 to 35°C, whereas in turkeys the maximum response was obtained at 70-75% rh. The maximum growth response in chickens coincides with minimum body and skin temperature. The arterial blood CO₂ partial pressure also exhibited a bell-shaped curve as a function of rh, with a peak at 60-65% rh, accompanied by the nadir of pH. In chickens, hyperthermia and respiratory alkalosis developed both at the high and low relative humidities. However, with the decline in T_a, the development of hyperthermia and respiratory alkalosis was moderate and the differences among treatments lost significance. In this species plasma triiodothyronine (T₃) concentration exhibits a positive correlation between T₃ and feed intake. Turkeys, however, respond differently to the increase in rh. They maintain T_b in the lower to moderate normothermic range for birds, despite severe changes in rh. Furthermore, turkeys maintain similar blood acid base balance between 40% and 75% rh, whereas only above 80% rh they do develop a severe respiratory alkalosis. A positive linear correlation between plasma T₃ and feed intake or weight gain is observed in turkeys. At low rh the differences in response between chickens and turkeys occur mainly as a result of the chicken`s difficulties in maintaining body water balance. At high rh both species have difficulties in dissipating heat by evaporation. The capacity of turkeys to maintain T_b strictly may be due to their efficient sensible heat loss. The relatively low tolerance of broiler chickens to heat stress can be improved by early age thermal conditioning (36±1.0°C, 70-80% rh for 24 h at the age of 3 to 5 days). This was tested during thermal challenge (36±1.0°C, 20-30% rh for 6 h at the age of 42 days). Thermal heat-conditioned chickens demonstrate a significantly low heat production, better capacity to maintain total body water, an efficient sensible heat loss and significantly low mortality. It may be concluded that: (a) Thermal conditioning induces changes in heat production and heat loss, which improves thermotolerance, but not to the level known in acclimated fowl. (b) The advantage of thermal conditioning for acclimated chickens lies in their better growth performance, (c) The changes in the threshold for heat production and/or heat loss seems to be the driving force behind the improved thermotolerance. (d) In the acclimated fowl the association between T_a and T₃ on the one hand, and between T₃ and feed intake and/or weight gain on the other hand, plays a major role in the development of the domestic fowl.