Objective To study the influence of pregnant rats' prenatal chronic stress (PS) on learning and memory of their offspring rats and its possible molecular mechanisms.Methods Pregnant females were individually restrained for 45 min 3 times a day during pregnancy from day 14 to day 21.Control pregnant females were left undisturbed in their home cages.The rat offsprings were randomly assigned to PS group or control group.Males and females were kept for the study separately.The learning and memory of the developing rat offspring in the Morris water maze were examined.The basal levels of corticosterone (COR) and adreno-cortico-tropic-hormone (ACTH) were analyzed by using radioimmunoassay.The Golgi-Cox impregnation technique was used to compare density and morphology of the CA1 hippocampal dendritic spines.Results The escape latency (EL) to find the platform in the control group was significantly less than that in the PS group in female rat offspring (F =4.533,P ＜ 0.05),and the difference was statistically significant on the 5th day (t =2.788,P ＜ 0.01).EL to find the platform in the control group was significantly less than that in the PS group in male rat offspring (F =6.101,P ＜0.05),and the difference was statistically significant on the second day (t =3.051,P ＜ 0.01).In the space exploration experiments of the water maze,the retention time observed for the control group and the PS group in the goal quadrant was similar(P ＞ 0.05).The basal levels of the serum COR in the PS group were higher than those in the control group of female rat offspring(t =3.658,P ＜ 0.01) and the basal levels of the serum ACTH in the PS group were higher than those in the control group of male rat offsprings(t =2.319,P ＜ 0.05).A simplified pattern was observed in the CA1 hippocampal dendritic spines in the PS group,showing a less extent of dendritic arborization and the density was significantly lower than that in the control group(t =-3.072,P ＜ 0.01).Conclusions Altered function of the hypothalamic-pituitary-adrenal axis in the offspring mediates the cognitive alterations observed following prenatal stress should to be associated with the lower density and simplified pattern of CA1 dendritic spines.