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Epidemiological and experimental data support the hypothesis of 'fetal programming', which proposes that alterations in fetal nutrition and endocrine status lead to permanent adaptations in fetal homeostatic mechanisms, producing long-term changes in physiology and determine susceptibility to later disease. Altered hypothalamic-pituitaryadrenal (HPA) axis function has been proposed to play an important role in programming of disease risk. The aim of the present study was to examine the effects of maternal nutrient restriction imposed during different periods of gestation on the HPA axis function in sheep, at different ages postnatal. Pregnant ewes were fed a 50% nutrient-restricted diet from days 0-30 (group R1, nZ7), or from days 31-100 of gestation (group R2, nZ7) or a control 100% diet throughout pregnancy, (Control, nZ8). Blood samples were collected at 10-day intervals from day 40 of gestation to term. Lambs were born naturally and fed to appetite throughout the study period. At 2, 5 . 5, and 10 months of age lambs were given an i.v. injection of corticotrophin-releasing hormone (CRH) and blood samples were collected at K15, 0, 15, 30, 60, 120, and 180 min postinjection. Maternal cortisol levels were significantly higher (P!0 . 05) in group R1 compared with the other two groups, whereas maternal insulin levels were lower (P!0 . 05) in group R2 compared with control. Birth weight of lambs was not affected by the maternal nutritional manipulation. The area under the curve for ACTH and cortisol response to CRH challenge was greater (P!0 . 05) in lambs of group R1 at two months of age, whereas no difference was detected at the ages of 5 . 5 and 10 months. However, significantly higher (P!0 . 01) basal cortisol levels were observed in lambs of R1 group at 5 . 5 months of age.There was no interaction between treatment and sex for both pituitary and adrenal responses to the challenge. A significant sex effect was evident with females responding with higher ACTH and cortisol levels at the age of 5 . 5 months (P!0 . 01, P!0 . 001 respectively) and with higher cortisol levels (P!0 . 01) at 10 months of age than males. It is concluded that the HPA axis is programmable by altered nutrition in utero. The sensitivity of the axis to exogenous stimulation is enhanced during early postnatal life and attenuated with age, suggesting a role for the postnatal influences in resetting of the HPA axis and emphasizing the importance of identifying the impact of maternal undernutrition at several time points after birth.