Age-appropriate acute stress, such as cold exposure, provokes the secretion of corticotropin releasing factor (CRF) from the hypothalamus, leading to a robust increase of plasma corticosterone in the immature rat. This activation of the hypothalamic-pituitary-adrenal system is accompanied by a stress-induced increase of steady-state CRF-mRNA expression in the hypothalamic paraventricular nucleus (PVN). In the current study, we analysed changes in CRFmRNA expression in the PVN and the central nucleus of the amygdala (ACe) in the immature rat in response to a single episode of cold stress and three repeated exposures to this same stressor. CRF-mRNA expression in the PVN increased after a single, but not repeated exposures to cold stress, while repeated acute stress increased the content of the CRF peptide in the anterior hypothalamus. In the ACe, repeated episodes of cold stress resulted in increased expression of CRF-mRNA. These findings indicate a differential regulation of CRF gene expression in the PVN and ACe of the immature rat by single and repeated acute stress. Keywordscorticotropin releasing factor; neonatal rat; hypothalamus; stress; amygdala The developing rat responds to environmental stressors by activation of the hypothalamicpituitary-adrenal (HPA) axis (1, 2). However, the mechanisms of this hormonal stress response may differ in the neonatal rat compared with the adult. The developmental period between the third and fifteenth postnatal days has been characterized by attenuated hormonal responses and altered gene regulation in response to stress (2-5).The neurohormone corticotropin releasing factor (CRF) activates both hormonal and behavioural responses to a variety of stressors. In both the mature and the developing rat, stress leads to CRF release from the hypothalamic paraventricular nucleus (PVN), resulting in increased plasma adrenocorticotropin (ACTH) and corticosterone concentrations. The stress-induced elevation of plasma corticosterone in the immature rat is dependent on CRF secretion as demonstrated using antisera to CRF (2). Subsequent to the stress-induced CRF secretion, a compensatory increase in CRF-mRNA expression in the PVN has been observed in the adult rat (6) as well as in the developing rat starting in the second week of life (2). CRF-containing cells constitute a significant neuronal population in the central nucleus of the amygdala (ACe). The ACe has been shown to be a key regulator of the stress response mediated by CRF release from the PVN [reviewed in (7)]. For example, ablation or stimulation of the ACe attenuate or mimic, respectively, the effects of stress on CRF release from the PVN (8, 9). However, the modulation of CRF gene expression in the ACe by stress has not been examined in the developing rat.The spectrum of differential alterations in the neuroendocrine axis of the adult rat by repeated stress compared with a single acute stress has been a topic of intense study. However, there is relatively limited information regarding the modulation of the neuroendoc...