The effect of repeated stress on corticotropin-releasing factor (CRF) and vasopressin (AVP) synthesis in parvocellular neurons of the paraventricular nucleus (PVN) was studied by means of double label immunocytochemistry. Once daily immobilization of male rats for 16 days leads to a 75% increase in the number of CRF immunoreactive neurons, but a 5-fold elevation in the number of AVP containing CRF cell bodies. These results demonstrate that repeated stress activates AVP synthesis in CRF neurons.