This study was designed to test the effect of pre- and post-hatch temperature conditioning or a combination of both, on the acquisition of heat tolerance during the adult life of broiler chickens. Nine hundred hatching eggs produced by Cobb broiler breeders were incubated at standard incubation conditions until d 16. Half of the eggs were subjected to temperature conditioning for 3 h/day (39.5 degrees C, 65% relative humidity (RH)) at d 16, 17 and 18 of incubation (T group) while the other half Control group (C group) were kept at standard incubation conditions. From the end of d 18 until end of incubation, embryo heat production (HP), gas partial pressure in the air chamber at IP, and blood parameters (T(3) and corticosterone) were measured. Also, hatching time for individual chick, body temperature (Tb) and body weight (BW) and the number of hatched chicks were recorded. Hatched chicks were raised under regular conditions. At the age of 3 d, the chicks of each incubation condition group were divided into 2 groups: half of the chicks of each group (C and T) were subjected to thermal conditioning (41.0 degrees C for 6 hours). The other half of the chicks was used as control. The 4 groups of broilers were reared until 42 d of age. During post-hatch period, Tb, blood parameters and BW were again measured. At 42 d all broilers were heat challenged at 35 degrees C for 6 h. After heat challenge, mortality was recorded and blood samples were collected. The results indicate that thermal conditioning during incubation had no effect on hatchability of eggs but prolonged incubation duration, decreased T3 (at IP), corticosterone (at IP and hatch), HP and Tb. Overall, at 3 d post-hatch, prenatal condition increased while post-natal conditioning decreased corticosterone levels. Heat challenge at 42 d post-hatch decreased T3 levels in the TC group and increased corticosterone levels in postnatally conditioned group. Differences between BW become obvious from 28 d post-hatch and were in the following order: TT > CC = CT > TC. At 42, the highest BW was obtained in the broilers of TC group. Heat conditioning at 3 d of age improved heat tolerance in response to heat challenge at 42 d. post-hatch whereas prenatal treatment had a strong negative effect. It is concluded that heat treatment during incubation or during post-hatch life induces completely different effects.