To specifically address the role of Rgs1 in the accumulation or emigration of monocyte-derived cells in the aortic wall during aneurysm development, we used a pulse-chase approach to track bead-labelled monocytes in aortas following Ang II infusion. Inflammatory 7/4 hi monocytes were labelled in vivo with fluorescent latex microbeads and administered intravenously (i.v.) at the time of osmotic mini pump implantation 23,24 . Aortic cell numbers were quantified by flow cytometry at days 3 and 5 after bead injection (Fig. 6a). At 3 days post Ang II infusion, the time of peak monocyte recruitment from the bloodstream, bead-positive leukocyte content in aortas was similar between ApoE À / À and Rgs1 À / À ApoE À / À mice, implying that monocyte recruitment was similar between the groups (Fig. 6b). However, by day 5 after the initiation of Ang II infusion, beadpositive CD45 þ cells were significantly higher in ApoE À / À aortas compared with Rgs1 À / À ApoE À / À mice, suggesting that RGS1 promotes the accumulation of ApoE À / À monocytes in aortic tissue, rather than emigration. Importantly, these findings likely underestimate the magnitude of accumulation of bead-labelled monocytes, since we found several ApoE À / À mice with aneurysms at the time of harvest, which were excluded from the analysis because of the confounding effect of blood cells trapped in aneurysms, to the flow cytometric analysis of aortic cells. Bead-labelled monocytes localized to areas of the subintimal space of the vessel wall and were 7/4 and Ly6C positive (Fig. 6c). In contrast to the difference in bead-labelled cells in the aortic wall, there was no difference in the number of circulating bead-labelled inflammatory monocytes between ApoE À / À and Rgs1 À / À ApoE À / À mice at day 3 or 5 ( Supplementary Fig. 9). Together these **data** suggest that RGS1 is a mediator of inflammatory monocyte accumulation in aortic aneurysms. 