A perspex model of a dog aortic trifurcation was machined to scale and perfused with steady flow from a constant pressure reservoir. The tail artery was plugged to produce a flow model of an intracranial saccular aneurysm. At all flow rates, no flow occurred beyond 2.5 tube diameters of the tail artery downstream from the mouth of the aneurysm. This was assumed to explain why large aneurysms thrombose. Measurements of velocity fluctuations were made with a hot film anemometer and recorded on tape. Frequency analysis showed that the peak frequency was a function of flow rate, and suggested that eddies were shed from the origin of the aneurysm. This was presumed to be an artifact due to sharp entrance produced by machining the perspex. The total energy at any one point in the aneurysm was independent of the size of the aneurysm but increased with flow rate. The maximum fluctuations were comparable in the center and in the sides of the aneurysm, but were less on the top and bottom of it (assuming the central plane was in the plane of the trifurcation). This difference presumably would be less if the aneurysm were spherical rather than cylindrical.