A thorough understanding of the physiological consequences of traumatic blood loss is essential to effective medical management of the problem and the associated complications. The response to blood loss in conscious animals (including humans) is biphasic. The pattern is characterized initially by sympathoexcitation and arterial blood pressure maintenance with little activation of humoral systems. This is followed later by sympathoinhibition, vasodilation, hypotension, and increased release of vasopressin, epinephrine and renin. Very little is actually known about the central nervous system networks controlling the onset of hypotension. The periaqueductal gray (PAG) is a brain area that may be important in the control of the cardiovascular and neurohumoral response to blood loss. Our general hypothesis is that the ventrolateral PAG is important in the central control of the integrated hemodynamic and neurohumoral response to hemorrhage and stressful sensory stimuli. We will use a combination of approaches to evaluate ventrolateral PAG involvement in cardiovascular control during acute hypotensive hemorrhage in conscious rabbits. The use of this combination of approaches is novel, and this will be one of the first studies to address ongoing central cardiovascular control mechanisms during a physiological event such as hemorrhage. The Specific Aims of this study are: 1) To define the role of the ventrolateral PAG in the hemodynamic response to blood loss and stress in the conscious rabbit. 2) To quantify the response of ventrolateral PAG neurons during hemorrhage and stress in the conscious rabbit. 3) To understand the role of serotonergic and opioidergic mechanisms in the ventrolateral PAG during blood loss in the conscious rabbit.