Neuromedin B (NMB) is a bombesin-like peptide that exerts its function via NMB receptor (NMB-R). The NMB/NMB-R pathway is involved in the regulation of a wide variety of behaviors, such as spontaneous activity, feeding, and anxiety-related behavior. In the current study, we assessed the effects of stress on maternal behavior in female NMB-R-deficient mice. Non-stressed NMB-R-deficient and wild-type mice showed normal maternal behavior. However, immediately after undergoing restraint-induced stress (30 min) both genotypes of mice exhibited severely decreased maternal behaviors. Furthermore, 30 min after stress induction, maternal behavior in wild-type mice recovered to near normal levels whereas that of NMB-R-deficient mice remained significantly lower. These results indicate that NMB-R-deficient mice suffer more severely from stress and suggest that dysfunction in the NMB/NMB-R pathway may constitute one of the risk factors of stress vulnerability.