Comparison of the hypothalamic pituitary adrenal (HPA) system reactivity between germ-free (GF) and conventional (SPF) F344 rats after an open-field (OF) stress. A greater elevation of systemic corticosterone occurred in GF OF-stressed rats than in the SPF counterparts. This was accompanied by a greater corticotropin releasing factor (CRF) mRNA expression level in the hypothalamus and a lower glucocorticoid receptor (GR) mRNA expression level in the hippocampal CA₁ area. Data are means with sem (n = 8 to 12 rats/group). ** p< 0.01, *** p< 0.001.