Abstract 1. A sudy was conducted to test the hypothesis that feed efficiency correlated with the expression of genes from the somatotropic axis and mitochondrial genes involved in energy production, and that the environment to which the birds are exposed influenced the expression of such genes. 2. Quails were divided into high feed efficiency (High-FE) and low feed efficiency (Low-FE) groups and maintained in a comfortable or heat stress (38ºC for 24 h) environment to evaluate changes in insulin-like growth factor I (IGF-I), growth hormone receptor (GHR), adenine nucleotide translocator (ANT), uncoupling protein (UCP) and cytochrome oxidase subunit III (COX III) mRNA expression in liver and muscle tissues. 3. High-FE quails (0.28 g/g) presented a higher final body weight, greater weight gain, and a better feed conversion ratio than low-FE birds (0.18 g/g). High-FE birds showed greater IGF-I mRNA expression in the liver and muscle and greater GHR mRNA expression in the muscle. 4. Environmental effects only affected GHR expression in the liver, with quails under comfortable conditions exhibiting greater GHR expression than quails subjected to heat stress. 5. There was a significant interaction between feed efficiency and environmental temperature on ANT mRNA expression in the liver. The greatest ANT mRNA expression was observed for high FE-birds that remained under comfortable conditions. 6. In the liver, UCP mRNA expression did not differ among the quails and was not affected by environment or efficiency. However, comparisons of the low- and high-FE birds revealed higher levels of UCP mRNA in the muscle of low-FE birds. 7. COX III mRNA expression in the liver was dependent on environmental temperature and feed efficiency. Higher COX III mRNA expression was observed in animals that remained under comfortable conditions, and high-FE birds exhibited higher expression levels compared to low-FE birds. 8. These results suggest a correlation between IGF-I, GHR, ANT, UCP and COX III gene expression and feed efficiency and that environmental temperature could affect the expression of some of these genes.