The plasma cortisol levels measured one hour after injection of ACTH reflects the hypothalamic-pituitary-adrenocortical (HPA) axis activity. The test was performed on piglets at 6 weeks of age. It was investigated as a potential selection criterion to improve pig robustness. From a base population of Large White pigs, two divergent lines were derived for 3 generations of selection, one selected for a high cortisol level (H line), one for a low cortisol level (L line). All breeders, as well as unselected animals, were also genotyped on a 70kSNP chip. At the 3rd generation of selection, the divergence between the two lines was about 5 genetic standard deviations. The high heritability of the trait was confirmed (h² = 0.64 ± 0.03). The genetic correlations with individual body weights and growth rates measured during lactation, post weaning and finishing periods were very low except for the genetic correlation between cortisol level and post-weaning growth rate (rg=-0.22 ± 0.10). The genome-wide association study revealed a quantitative trait locus influencing post-ACTH cortisol level, located on chromosome 2. It was suggested that divergent selection modified the frequency of NR3C1 alleles, favouring the hypersensitive glucocorticoid receptor in the low line.