To assess the role of neuromedin B receptor (NMB-R) on the modulation of serotonergic (5-HT) system, the function of the 5-HT system was examined in mice lacking the NMB-R gene. Immunohistochemical analysis of brain sections revealed that 5-HT expression level in the dorsal raphe neurons was elevated in NMB-R-deficient mice compared with wild-type mice. Although restraint stress enhanced 5-HT expression in these neurons in wild-type mice, this treatment did not affect 5-HT expression level in NMB-R-deficient mice, indicating the modulation of 5-HT system in the mutant mice. Since 5-HT system is involved in responses to stress and anxiety, we characterized stress response in these mice. The number of c-Fos expressing cells in the paraventricular nucleus of the hypothalamus was higher in NMB-R-deficient mice than in wild-type mice in both basal and stressed conditions. Moreover, the plasma corticosterone level under restraint stress was elevated in NMB-R-deficient mice compared to wild-type mice. In the forced swimming tests, the duration of immobility was longer in mutant mice than in wild-type mice. These data show dysregulated response to stress in NMB-R-deficient mice. However, behavior related to anxiety, assessed by elevated plus-maze and light-dark box, was not affected in NMB-R-deficient mice. NMB-R is known to be expressed in dorsal raphe neurons, and our data suggest that NMB-R has an important role in fine tuning of subsets of 5-HT neurons in this nucleus, and impairment of this system leads to the dysregulated response to stress.