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OBJECTIVE: This study looked at the effects of single and multiple small subarachnoid hemorrhage (SAH) caused by puncturing a small branch of the basilar artery in rats. METHODS: Rats were subjected to single SAH (n = 21), multiple SAH (n = 21), sham operation (n = 21), or no procedures (control group, n = 7). SAH was induced in rats by tranclival puncture of a small branch of the basilar artery. In the multiple-SAH hemorrhage-groups, three small hemorrhages were produced in the same artery at three different times (initial and 24 and 48 h). In the single-SAH groups, one small hemorrhage was produced. Measurements of local cerebral blood flow (LCBF) were made at the initial SAH procedure and at three different time points. Seven animals from each general grouping were killed on Days 4, 10, and 14 (after LCBF was measured). Three different levels of the basilar artery were examined in each animal. Luminal area and arterial wall thickness were measured, and the findings were compared with control and corresponding sham groups findings. RESULTS: LCBF dropped dramatically (by 40%) immediately after SAH and reached levels near baseline within 15 minutes (n = 42) (P < 0.001). LCBF continued to drop after initial SAH and reached the lowest level on Day 10 (P < 0.001) or Day 14 (P < 0.05). Significant luminal narrowing (P < 0.01) and thickening of the arterial wall (P < 0.01) were observed in both groups. CONCLUSION: Single or multiple small SAHs produced by puncturing the basilar artery in the rat cause similar acute and chronic cerebral vasospasm.