Vascular catheterization is increasingly carried out in laboratory mice, but the long-term patency of catheters implanted in mice is limited owing to their small size. The authors compared mice that were catheterized in their right common carotid artery either with a silicone catheter with a polyurethane tip or with a 100% polyurethane catheter to determine which catheter type was better suited for long-term studies in mice. The catheters were inspected daily and were flushed if blood was visible in the catheters; if no blood was visible, they were flushed every 3 d. Silicone catheters were patent for a shorter period of time than polyurethane catheters, which were patent for a median of 6 d and up to 25 d. The authors identify the principal causes of catheter failure and discuss how they can be prevented.