The present study utilized electrophysiological techniques to determine the effects of subfornical organ (SFO) stimulation on the activity of neurons in the paraventricular nucleus (PVN) projecting to the spinal cord. Single-unit recordings were obtained from 79 PVN neurons antidromically identified as projecting to the intermediolateral cell column (IML). Antidromically evoked action potentials showed a mean latency of 94.6 +/- 5.3 ms and a mean threshold for activation of 1.58 +/- 0.11 mA. Electrical stimulation of SFO (100 microA-1.5 mA, 0.1 ms) resulted in excitatory responses in 18 of the 27 neurons tested (67%). Peristimulus histogram analysis of such effects demonstrated a duration of < 50 ms in 14 of the 18 cells so influenced (78%), whereas the remaining 4 cells (22%) showed excitatory responses with a longer duration. Systemic administration of the nonpeptidergic angiotensin II (ANG) type 1 (AT1) receptor antagonist losartan (3 mg/kg) blocked the long-duration excitatory responses in 100% (3 of 3) of the cells tested but was without effect on the short-duration excitations (0 of 5). Twenty-two identified PVN neurons were also tested for their responses to systemic ANG (20-500 ng), which had no observable effect on the activity of any of these cells. These data demonstrate that neurons in SFO provide excitatory input to PVN cells that project to the IML. One of the neurotransmitters responsible for communication in this pathway is ANG.