SUMMARY A method for induction of subarachnoid hemorrhage (SAH) in a rat model is described. Resolution of the hemorrhage was documented photographically and microscopically at intervals from 1 hr to 8 days. Photographs indicated that most of the hemorrhage was resorbed within 3 days, an observation confirmed microscopically by the amount of red blood cells in the subarachnoid space. Significant cerebral vasospasm was documented within the first 2 days after the induction of hemorrhage with the basilar artery returning to baseline values at an average of 3 days followed by moderate dilatation at 5 to 8 days. 10 -u The possibility of using a smaller, less expensive laboratory animal has not been thoroughly investigated. The present investigation sought to determine whether the rat was a suitable animal for the study of SAH with regard to the mode of SAH induction, its time course of resolution and the degree of cerebral vasospasm produced. Materials and MethodsAll animals were 400-600 g male Sprague-Dawley rats. They were anesthetized with intraperitoneal injections of sodium nembutal (40-50 mg/kg). The rats were placed in the supine position, the ventral portion of the neck shaved, and a midline incision approximately 2.5 cm in length made. The trachea was exposed and an endotracheal tube (ID 0.062", OD 0.082") inserted orally, and visualized entering the exposed trachea. Repeated suction ensured maintenance of an adequate airway. Using an operating microscope, the cervical musculature and carotid arteries were retracted to expose a 15-20 mm 2 area of clivus. A midline craniectomy (less than 2 mm 2 ) was performed using a #23A Horico diamond burr to expose the normally transparent dura and underlying basilar artery. A tungsten microelectrode, drawn to a 40-60 micron tip, was positioned over the exposed basilar artery using a stereotactic device ( fig. 1). The tip was advanced through the intact dural and arachnoid membranes and into the lumen of the basilar artery. SAH was induced upon slow withdrawal of the tungsten tip. The subarachnoid progression of the hemorrhage was visualized microscopically through the craniectomy. Gelfoam® (absorbable gelatin sponge, The Upjohn Company, Kalamazoo, MI) was placed at the craniectomy site and the neck incision closed with skin clips. The endotracheal tube was left in place approximately 1-2 hr postoperatively until the animal had recovered from the effects of the anesthesia and was without obvious respiratory distress.The site of the SAH was reexposed 1 hr (acute), or 1, 2, 3, 5, and 8 days postoperatively. Animals used acutely were maintained on sodium nembutal postoperatively. After recovering from initial surgery, all chronic animals were reanesthetized, tracheotomized, and intubated. The clivus was reexposed and the craniectomy enlarged with the diamond burr revealing a larger segment of the basilar artery (approximately 7-10 mm). The dura was then carefully removed allowing better visualization of the underlying basilar artery and brainstem. Photographs were taken thr...