A QTL analysis of behavioral and neuroendocrine responses to a "novelenvironment" stress was conducted in a three-generation experimental crossbetween Meishan and Large White pig breeds. A total of 186 F2 males and 182 F2females were studied for their behavioral and neuroendocrine reactivity to anovel environment test at 6 wk of age. Locomotion, vocalization, and defecationrate, as well as exploration time, were measured for 10 min. Blood samples weretaken immediately before and after the test to measure plasma levels of ACTH,cortisol, and glucose. Animals were typed for a total of 137 markers coveringthe entire porcine genome. Analyses were performed using two interval mappingmethods: a line-cross regression method, where founder lines were assumed to befixed for different QTL alleles, and a half-/full-sib maximum likelihood methodwhere allele substitution effects were estimated within each half-/full-sibfamily. Both methods revealed a highly significant gene effect for poststresscortisol level (P < 0.001) and a significant effect for basal cortisol level (P< 0.05) at the end of the q arm of chromosome 7, explaining, respectively, 20%and 7% of the phenotypic variance. Meishan alleles are associated with highercortisol levels and are partially dominant (for poststress levels) over LargeWhite alleles. Other significant gene effects on biological measures weredetected on chromosomes 1 and 17 (ACTH response to stress), 3, 5, and 8 (glucoselevels). The SSC 17 QTL explains 12% of the phenotypic variance of poststressACTH levels, with a suggestive evidence of imprinting effects. Meishan allelesare associated with lower poststress ACTH levels. Gene effects of low amplitudeonly were found for behavioral reactivity traits. Considering the effects ofstress neuroendocrine systems on energy fluxes and protein deposition, and theimportance of stress reactivity for meat quality and animal welfare, theseresults open new perspectives for pig selection.