In the rat, evidence now suggests a neurotransmitter function for the neuropeptides arginine vasopressin (AVP) and corticotropin releasing factor (CRF), implicating them in various autonomic, behavioral, and neuroendocrine responses to stress. Repeated AVP/CRF release in the pituitary portal circulation, due to stress, sensitizes and potentiates the release of ACTH from the anterior pituitary. Using a neuroanatomically well-defined behavior, the acoustic startle reflex in the rat, we sought to determine whether an interaction between AVP, CRF and stress might also occur centrally as measured by increased behavioral sensitivity to AVP or CRF given directly into the brain. The first experiment tested whether repeated intraventricular (i.c.v.) infusion of AVP would lead to an increase in the excitatory effect of a subthreshold dose of AVP on the acoustic startle reflex when infused 48 h later. Different groups of rats were infused with various doses of AVP (0.3, 3, or 30 ng) or vehicle on Day 1 and tested for startle over the next 60 min. On Day 2, 48 h later, all animals were infused with a single dose of AVP (300 pg) and tested for startle. Infusion of AVP on Day 1 did not increase startle consistently at any dose, but did lead to a sensitized excitatory effect of AVP on startle on Day 2 which was non-monotonically related to the dose of AVP given on Day 1. Experiment 2 tested whether AVP on Day 1 would sensitize the excitatory effects on startle of CRF given i.c.v. on Day 2. Different groups of rats were infused i.c.v. with various doses of AVP (10, 30, 100, 300 pg) or vehicle on Day 1. On Day 2, 48 h later, all rats were infused with a subthreshold dose of CRF (0.25 microg). Infusion of AVP on Day 1 led to a sensitized excitatory effect of CRF on startle on Day 2 which was non-monotonically related to the dose of AVP given on Day 1. In experiment 3, we tested whether footshocks given on Day 1 would sensitize the excitatory effect of CRF on startle tested 48 h later. Different groups were given footshocks (0.2, 0.4, 0.8, 1.6 mA) on Day 1. On Day 2, 48 h later, all rats were infused with a subthreshold dose of CRF (0.25 microg). Footshocks given on Day 1 led to a sensitized excitatory effect of CRF on startle on Day 2 which was non-monotonically related to the intensity of footshock on Day 1. Taken together, these results suggest that an interaction between AVP, CRF and stress may occur centrally, consistent with other studies showing similar interactions peripherally. This may provide a model system for analyzing how prior stress leads to enhanced behavioral reactions to subsequent stressors and a mechanism to explain dysregulation of the stress response.