Mean arterial pressure and drinking were measured in rats before and after intracranial injection of acetylcholine (ACh) into the subfornical organ (SFO), the circumventricular organ of the dorsal third ventricle. Arterial pressure was recorded in unanesthetized unrestrained animals by means of an abdominal aortic catheter. Maximal increases in arterial pressure and drinking followed the application of ACh to SFO, while significantly smaller effects were obtained after ACh injections into near-SFO control sites. Inasmuch as the effects also occurred with the shortest latencies after SFO injections, the possibility of ACh diffusing to act at other than SFO sites seems unlikely. The pressor and drinking responses to ACh were blocked by SFO pretreatment with atropine, indicating muscarinic receptor involvement. The SFO-mediated pressor response to ACh was reduced after systemic injection of phentolamine, suggesting at least a partial role for the sympathetic nervous system in the response. The present experiments suggest that ACh acts at the SFO to generate not only drinking but also increases in arterial pressure.