Wild-derived mouse strain MSM/Ms (MSM) still shows wildness behaviors which had been lost in laboratory mice during domestication. The wildness behaviors are considered as strong behavioral responses to stress. Our previous study revealed that the behavioral responses to stress in MSM mice are mapped on Chr17 under the genetic background of C57BL/6 (B6). In this study, 1) we conducted fine genetic mapping, 2) and characterized a candidate gene. 1) We established a series of sub-consomic strains, which carry partial segment of MSM-derived Chr17. In the results of genetic analyses of the behavioral response to stress in open field using the sub-consomic strains, we successfully mapped a genetic locus into about 2.3 Mb region at the distal end of Chr17, in which only one protein-coding gene PACAP is located. The PACAP gene was considered as candidate gene of the behavioral response to stress because PACAP is a neuropeptide which regulates stress responses. 2) Although non-synonymous mutation was not found in the PACAP gene between MSM and B6 strains, the PACAP mRNA and protein levels were significantly increased in hypothalamus of the sub-consomic strain. In the sequence comparisons, dinucleotide GT-repeat polymorphism was found in second intron of the PACAP gene in MSM strain. Further luciferase assay demonstrated that the GT-repeat polymorphism of MSM can increase the PACAP expression in PC12 cells. These increased PACAP expression levels were correlated with increased serum corticosterone level after restrained stress in the sub-consomic strain. Our findings indicate that variation in the behavioral response to stress in MSM mice is linked to the genetic variation in PACAP gene.