The circumstances surrounding aneurysmal subarachnoid hemorrhage were investigated in a group of 500 consecutive patients admitted to a neurosurgical center. Subarachnoid hemorrhage occurred during stressful events in 42.8% of the patients, during nonstrenuous activities in 34.4%, and during rest or sleep in 11.8%. The activities or events preceding subarachnoid hemorrhage were not known in the remaining 11.0%. Men were more likely to have suffered their hemorrhage during stressful events than women (54.1% versus 36.6%; p less than 0.00025). Only 30.1% of aneurysms arising from the internal carotid artery ruptured during stressful events compared with 48.1% of aneurysms at other locations (p less than 0.005). Physiological responses to the various activities and events are discussed as they may relate to mechanisms underlying aneurysmal subarachnoid hemorrhage. Two important factors in the precipitation of aneurysmal rupture are increased arterial blood pressure and decreased cerebrospinal fluid pressure around the aneurysm. These factors result in a high transmural pressure and concomitant wall stress. Physical exertion, activities involving the Valsalva maneuver, and emotional strain are associated with an increase in blood pressure and frequently preceded subarachnoid hemorrhage. Fluctuations in cerebrospinal fluid pressure around the aneurysm mainly occur during Valsalva's maneuver and postural changes.