Eighteen hundred eggs obtained from Ross broiler breeders at 32 and 48 wk age were randomly assigned to two incubation treatments: T1 eggs were incubated at 37.6 degrees C throughout, while for T2 eggs the incubation temperature was reduced 1 degrees C for 6 h daily at embryonic ages (EA) 10 to 18. Embryo and organ weights and body composition were measured at EA 14, 19 and day of hatch (DOH). Hatchability and hatching duration, as well as serum triiodothyronine (T-3), thyroxin (T-4), and triglycerides were measured at DOH. T1 eggs contained less water than T2 eggs at EA 18. Hatchability was lower and the incubation period was 4.2 h longer for T2 than T1 chicks. On DOH for older breeders chick weights and serum T-3 levels were higher for T2 than T1; however, those from younger breeders were similar at both incubation temperatures. These results may show a beneficial effect of T2 treatment in older breeders. Incubation temperature did not affect triglycerides levels. On DOH, higher body lipids content of T2 than T1 chicks may contribute to their resistance to cold post hatch.