The ontogeny in the rat hypothalamus of arginine vasopressin (AVP) and corticotropin-releasing factor (CRF) was studied to determine whether these two peptides develop in parallel, given the synergistic effect of AVP with CRF on adrenocorticotropic hormone (ACTH) release. Hypothalami and extrahypothalamic cerebrum of 15-day fetal--40-day postpartum rats were extracted in acetic acid for radioimmunoassay. Both AVP- and CRF-like immunoreactivity was detected in whole brain extracts of 15-day fetuses. Hypothalamic CRF levels were low during fetal life, fluctuated around the time of birth, and then progressively rose to 63% of adult levels by day 21 postpartum. Extrahypothalamic levels of brain CRF paralleled hypothalamic levels. Hypothalamic AVP levels began to rise at fetal day 19 in a stepwise manner, with the greatest rise in levels occurring after day 14 postpartum. At day 21 postpartum hypothalamic levels were only 17% of adult. Extrahypothalamic AVP levels initially paralleled those in the hypothalamus but reached a plateau after day 4 post-partum, and did not rise again until after day 21. Hypothalamic CRF and AVP thus do not develop strictly in parallel; the adult ratios of these peptides (approximately 1:40) were never seen during development, perhaps of significance in the observed blunting of the hypothalamic-pituitary-adrenal response to stress in infant rats.