Cellular levels of vasopressin (VP) and corticotropin-releasing factor (CRF) mRNAs were determined in the rat hypothalamic paraventricular nucleus after acute immobilization stress. Messenger RNA levels were measured by in situ hybridization histochemistry using 35S-labeled synthetic DNA oligonucleotide probes and quantitative autoradiography. Analysis of histograms related to density values revealed that two hours after immobilization the average cellular VP and CRF mRNA levels were significantly increased (145 +/- 30.3 and 68 +/- 21.3%, respectively, above control values). Moreover, the number of VP-expressing parvicellular neurons was doubled in stressed rats relative to controls. These results indicate that single acute immobilization stress is sufficient to up-regulate VP as well as CRF mRNA production in the hypothalamic CRF-neurosecretory system.