Early-age thermal conditioning (TC) by exposing young chicks to 40 C for 24 h reduces body temperature (Tb) and has been showed by others to improve long-term resistance of broilers to heat stress. Uncoupling oxidative phosphorylation in pectoral muscle mitochondria might be related to heat production. Fertile eggs were hatched under video control, and 161 pedigree chicks froml2 sires and 22 dams were immediately allocated to two groups (T, a group composed of 81 chicks exposed to TC at 5 d of age, and N, a control group of 80 nonexposed chicks). Body weights and Tb were measured at 2 and 7 d of age. Five pairs (one N and one T) of full sib chicks from families that exhibited the largest difference of Tb variation from 2 to 7 d of age between the two treatments were chosen for pectoral muscle sampling. Avian uncoupling protein (avUCP) messenger RNA expression was measured by reverse transcript-PCR coupled to southern blot in the pectoral muscle of 7-d-old broiler chicks. At 7 d of age, there were no BW differences between treatments and Tb was significantly reduced by TC (-0.13 C on average). Heritability of Tb variation between 2 and 7 d was 0.38 +/- 0.20 (SE) for T chicks and 0.35 +/- 0.17 for N chicks without a significant genetic correlation between the two environments. Expression of avUCP mRNA was significantly (85%) lower in T chicks than in N chicks. Uncoupling protein mRNA expression in pectoral muscle and Tb are quickly adjusted in broiler chicks 24 h after early thermal conditioning.