The present experiment was conducted to examine the effects of pre- and postnatal conditioning to induce thermotolerance in broilers hatching from eggs orginating from younger and older breeder flocks. From each flock, 500 eggs were randomly divided into 2 groups and incubated at standard (SIT) and high (HIT) temperatures. At hatch, chicks were allotted to 36 floor pens. At 5 d of age 3 pens per parent age per incubation temperature were heat conditioned (C) for 24 h. At 21 d of age, 3 pens per parent age per incubation temperature were kept under standard (S) rearing temperatures, whereas the remaining broilers (including C) were moved to a heated room (H). Thus, there were 3 groups from 21 to 49 d including S, H, and C+H. The results indicated that to incubate eggs from younger parents at HIT before conditioning at 5 d may reduce deleterious effects of heat stress on body weight at slaughter age. However, HIT temperature decreased slaughter weight of broilers from older parents regardless of rearing temperature. Although conditioning did not prevent increments in rectal temperatures of broilers after 1 wk of heat stress (28 d), adaptation occurred thereafter for broilers from younger parents but not for broilers from older parents. Incubation temperature had no effect on plasma glucose, creatine kinase, uric acid, or triiodotryronine (T3) levels at d 21. There were no differences at 22, 25, or 28 d for T3 concentrations between the H and C+H groups. Overall relative asymmetry was not influenced by treatments on d 49. The results suggest that although pre- and postnatal conditioning may help broilers cope with heat stress, age of parent plays a major role in the ability of broilers to thermoregulate. This is especially the case for broilers originating from younger parents.