These studies investigate the regulation of the HPA axis by endogenous amino acid transmitters acting through NMDA receptor in the late gestation fetal sheep. The HPA axis responsiveness to neuropeptide Y (NPY), a neurotransmitter implicated in metabolic feedback to the HPA axis, is also investigated. In the chronically cannulated late gestation sheep fetus CGP 37849, a competitive NMDA receptor antagonist, significantly decreased the ACTH and cortisol response to insulin hypoglycaemia stress on day 130(term=145 days), and also reduced basal plasma concentrations on day 138, confirming that endogenous excitatory amino acid neurotransmitters acting through the NMDA receptor regulate ACTH secretion in the late gestation fetus. The coupling of the NMDA receptor to CRH and AVP secretion at the median eminence was investigated by a microdialysis approach. Pre-treatment with CGP 37849 attenuated the ACTH and cortisol responses to insulin hypoglycaemia, but not AVP secretion at the median eminence. It was not possible to detect CRH in the dialysate from any of the fetuses, but the failure of CGP 37849 to suppress AVP release suggests that NMDA receptors regulate ACTH secretion through CRH neurons. The potential for placental steroids to regulate the HPA axis response to NMDA was also investigated. Infusion of estradiol from day 120 for 96 h did not alter the basal plasma ACTH or cortisol concentrations, pituitary sensitivity to AVP and CRH, or ACTH or cortisol responses to NMDA, despite there being a significant suppression of plasma follicle stimulating hormone concentrations and an increase in plasma prolactin concentrations. The role of NPY pathways to stimulate the HPA axis was also examined. In fetal sheep at day 125 gestation administration of 30 μg NPY into the lateral cerebral ventricle stimulated a small but significant increase in plasma concentrations of ACTH. Overall, these studies suggest an important role for the NMDA receptor acting through CRH neurons in the late gestation increase in fetal HPA axis activity, however, the increase in HPA axis activity and sensitivity to NMDA are not dependent upon estrogen. Activation of the HPA axis may be due to metabolic signals since in late gestation NPY pathways can stimulate the HPA axis.