Recent studies have focused on the serotonergic mechanism mediated via serotonin (5-HT) receptors underlying regulation of emotional stress during the developmental period. The present study was undertaken to elucidate whether early postnatal stress affects rat brain development and influences the serotonergic function in the midbrain median raphe nuclei (MRN) and dorsal raphe nuclei (DRN) in the adult, focusing on the response to unconditioned fear stress. Rats received aversive foot shock (FS) stimuli at the third week of the postnatal period (3wFS), but not those at the second week (2wFS), had increased percentage of time spent on open arms, estimated by the elevated plus maze test, at the postadolescent period (10-12 weeks old). The anxiolytic behavior observed in 3wFS was similar to that in rats having electrolytic lesion of the MRN, but not the DRN. In addition, the number of MRN 5-HT-immunoreactive cells in 3wFS remarkably was reduced compared to the non-FS control and 2wFS groups. These data suggest that aversive stress at the third week is attributable to the serotonergic function in the MRN underlying regulation of unconditioned fear stress. In other words, the "critical period" appears to be the time of neural circuit development of the MRN serotonergic system, which may be implicated in lifelong susceptibility to emotional stress.