We investigated the response of the fetal pituitary-adrenal axis to acute and chronic hypoglycemia before and after the normal prepartum activation of this axis at around 135 days gestation (term = 147 +/- 3 days). Pregnant ewes were either well nourished (control group; n = 22) or undernourished (UN; 50% reduction in maternal nutrient intake; n = 23) during the last 30 days of pregnancy. Acute hypoglycemia was induced by intrafetal administration of insulin between 125 and 130 days gestation (control, n = 7; UN, n = 12) and between 138 and 141 days gestation (control, n = 6; UN = 9). Fetal plasma glucose concentrations were significantly lower (P < 0.005) in the UN compared with the control group throughout the insulin infusion period at both gestational age ranges. In the control group, there was no fetal ACTH response to insulin infusion before 135 days gestation, but there was a significant (P < 0.001) response after 136 days gestation. In the UN group, there was a significant ACTH response to insulin infusion both before and after 135 days gestation, and there was no difference in the fetal ACTH response between the two gestational age ranges. The plasma cortisol responses to insulin were greater (P < 0.001) after 136 days compared with before 135 days gestation in both the UN and control groups. In the control group there was no significant relationship between basal fetal plasma ACTH and glucose concentrations between 115-135 days gestation or between 136-145 days gestation. In the UN group, fetal glucose ranged from 0.5-2.0 mM, and plasma ACTH and glucose concentrations were inversely related at 115-135 days gestation [log ACTH = -0.31 (glucose) + 2.21; r = -0.37; P < 0.001] and at 136-145 days gestation [log ACTH = -0.40 (glucose) + 2.50; r = -0.54; P < 0.001]. When the UN and control groups were combined, fetal plasma ACTH concentrations were significantly greater (F = 13.5; P < 0.05) when plasma glucose concentrations were less than 1.0 mM at either 115-135 days or 136-147 days gestation. Similarly, fetal plasma cortisol concentrations were also significantly greater (F = 18.7; P < 0.05) when plasma glucose concentrations were less than 1.0 mM at each gestational age range. Therefore, there is an increased sensitivity of the fetal hypothalamo-pituitary axis to acute falls in glucose concentrations below 1.2 mM after 135 days compared with earlier in gestation. The fetal hypothalamo-pituitary axis can respond, however, when plasma glucose concentrations fall below 1.0 mM, before and after 135 days gestation, independently of whether the low glucose concentrations are a consequence of insulin-induced hypoglycemia or maternal nutrient restriction.