The importance of the hypothalamus in regulating normal arterial pressure has been established by a number of investigations. Recent studies have implicated altered hypothalamic neuronal function in some forms of hypertension. However, the precise nature of hypothalamic control of arterial pressure has not been clearly established. Recent advances in neuroanatomical tracing techniques suggest that the paraventricular nucleus (PVN) may play a major role in central cardiovascular regulation through descending projections to the medulla and spinal cord. These include, among others, mono-synaptic projections to (1) the intermediolateral cell column (IML), the site of preganglionic sympathetic neurons in the spinal cord, (2) the dorsal motor nucleus of the vagus (DMNV), and (3) the nucleus tractus solitarius (NTS), the site of primary cardiovascular afferent termination. The objectives are to begin a neurophysiological and pharmacological description of the function of descending projections from the PVN in controlling arterial pressure by using several neurophysiological techniques. Initial studies will characterize the effects of stimulation and lesion of the PVN on arterial pressure and heart rate in rats. To further understand this system, the influence of PVN stimulation on the activity of identified IML, DMNV, and NTS neurons showing putative cardiovascular-related activity will be examined. These experiments will be combined with the microinjections and micropressure application of putative neurotransmitters suspected to comprise the descending pathways from the PVN, particularly vasopressin and oxytocin. These two transmitters are thought to comprise a major portion of the descending projections to the medulla and spinal cord. Intracellular recording studies will be conducted to elucidate the postsynaptic effects of these transmitters and the nature of synaptic input from PVN to cells in these regions. The objective of this and succeeding studies is to elucidate the effects of hypothalamic projections upon sympathetic and parasympathetic preganglionic neurons, and other medullary neurons involved in central neural regulation of arterial pressure.