Genetic background affects animal phenotype and therefore is of particular relevance to studies using genetically manipulated mice. Strain differences in hypothalamic–pituitary–adrenocortical (HPA) axis activity may contribute to background-specificity of some mutations. Here, we analysed components of the HPA axis in mice lacking a functional neurokinin-1 receptor (NK1–/–) on two backgrounds: backcrossed C57BL/6 (B6) and mixed C57BL/6 × 129/sv (129B6). We hypothesized that HPA axis activity would vary between these strains, leading to differences in the NK1–/– phenotype. We compared levels of plasma corticosterone between the groups, and found 129B6 mice exhibited elevated levels of stress-induced corticosterone compared with B6 mice, regardless of genotype. Although the level of basal corticotrophin-releasing factor and stress-induced c-fos mRNAs did not differ between the genotypes of either strain, examination of glucocorticoid receptor immunoreactivity within the hippocampus revealed that NK1–/– mice on the 129B6 background had elevated expression compared with wild-type, whilst there was no difference between genotypes in the B6 strain. Similarly, hippocampal neurogenesis in NK1–/– mice was greater than in wild-type on the 129B6 strain, and did not differ between genotypes on the B6 background. Finally, novelty- and morphine-induced locomotion were assessed. NK1–/– mice on the 129B6 background exhibited hyperlocomotion in response to novelty and greater sensitivity to the locomotor-stimulating properties of morphine than wild-type. In contrast, in B6 mice, no differences were observed between genotypes for either locomotor behaviour. In summary, we find that HPA axis activity differs between the strains and that there are profoundly background-specific effects of the NK1 receptor mutation.