This study was undertaken to determine whether microemboli injected in a predetermined sequence would maintain that sequence once they came to rest in brain microvessels. If so, the injection of different-colored microspheres at different times could be used to bracket-in-time emboli that are known to be released into the circulation during cardiopulmonary bypass. We injected different-colored microspheres into the arterial circulation of anesthetized dogs before and after the injection of fat emboli and before and after cardiopulmonary bypass. Coronal slices of the dog brains were embedded in celloidin, sectioned at 100 microns, and stained for alkaline phosphatase. The afferent cerebrovasculature stained dark brown against a light background, and the proximal/distal orientation of many of the arterioles could be determined by following their course within the thick sections. When different types of emboli were found in a single arteriole, they appeared in the order injected or the order of occurrence in the bypass protocol in 99.3% of the 867 such arterioles counted. Therefore, the microemboli maintained their ordered sequence with only a very small degree of mixing. Once they came to rest, there was not sufficient collateral blood flow in the brain microvessels to move them into disordered positions. This dog model should facilitate studies of the time of release of microemboli within narrower windows of time during cardiopulmonary bypass.