1. Rats underwent either: (1) acute or chronic morphine or naloxone administration; (2) simple morphine withdrawal or naloxone-precipitated withdrawal in morphine-dependent animals; or (3) stress from I.P. administration of hypertonic saline. 2. Quantitative in situ hybridization histochemistry was performed using synthetic oligonucleotide probes for corticotrophin-releasing factor (CRF), vasopressin, pro-opiomelanocortin (POMC), dynorphin, enkephalin and oxytocin mRNAs. The paraventricular and supraoptic nuclei were examined in all studies and the arcuate nucleus and pituitary gland in the acute withdrawal study. 3. Neither acute nor chronic morphine administration altered either (a) hypothalamic parvocellular or magnocellular CRF mRNA, or (b) anterior pituitary or pars intermedia POMC mRNA. 4. Naloxone-precipitated morphine withdrawal resulted in a marked increase in parvocellular (but not magnocellular) CRF mRNA within 4 h and levels remained elevated through 24 h. There was no change in arcuate nucleus or pars intermedia POMC mRNA, but in the anterior pituitary there was a delayed increase, significant at 24 h. 5. Simple morphine withdrawal without the use of naloxone did not result in any change in CRF mRNA but there were increases in magnocellular vasopressin and dynorphin mRNA, presumably related to decreased water intake. 6. Intraperitoneal hypertonic saline stress also resulted in a marked accumulation of both parvocellular CRF and vasopressin mRNA without any concomitant change in magnocellular vasopressin mRNA. Increased translation of CRF mRNA was also evidenced by increased immunoreactive CRF detected by immunocytochemistry.