The ventrolateral periaqueductal gray (vlPAG) has been proposed as a site responsible for the active process triggering the onset of hypotension (i.e. phase 2) during blood loss in conscious animals (4). We recorded the extracellular activity of PAG neurons in conscious rabbits to test the hypothesis that vlPAG neurons change their firing frequency prior to the onset of hypotension during simulated hemorrhage. Arterial and venous catheters, an intrathoracic vena caval occluder, and midbrain microelectrodes on a microdrive were implanted in ten rabbits. During simulated hemorrhage the occluder was inflated until arterial pressure 40 mmHg. We compared changes in neuronal activity during simulated hemorrhage to those during a similar length control period for 64 vlPAG and 29 dorsolateral (dl) PAG neurons. Arterial pressure pulse modulation of neuronal activity was present in 45% and 76% of vlPAG and dlPAG neurons, respectively. When we evaluated the absolute change in activity, thus accounting for both increases and decreases, simulated hemorrhage had a significant effect on activity of vlPAG, but not dlPAG neurons. The majority (56%) of vlPAG neurons did not appear to respond to simulated hemorrhage. Of the 28 responsive vlPAG neurons: 11 showed an abrupt change in firing frequency during the time interval preceding the onset of hypotension; 13 responded after the onset of hypotension; and 4 showed a consistent direction of change across the entire simulated hemorrhage. Thus, 24 (38%) of the vlPAG neurons recorded responded at a time consistent with a contribution to the hypotension associated with simulated hemorrhage.