Cases involving massive blood transfusion may require the use of specialized blood warmers, such as the Level 1 (L-1) (Level 1 Technologies, Inc., Rockland, MA) or the Rapid Infusion System (RIS) (Haemonetics Corp., Braintree, MA). In this in vitro study, we compared the infusion and warming capabilities of the L-1 (model 1000) versus the RIS using pediatric- and adult-sized IV catheters. The time to infuse 2 L of lactated Ringer’s solution and the end temperature after infusion through 20-, 18-, 16-, and 14-gauge catheters, and 4-, 5-, 6-, 7-, and 8.5-French catheters using both the L-1 and RIS were measured. The flow rates of both systems were similar for 18- and 20-gauge catheters; however, the flow rates with the RIS were progressively faster than the L-1 as catheter size increased to >18 gauge. The heating capabilities of the RIS were superior to the L-1 for all catheters ≥16 gauge. We conclude that the RIS was superior to the L-1 for both flow rates and warming capacity for all IV catheters >18 gauge, i.e., those used for cases with massive blood loss. The RIS provided no advantage (with regard to heating and flow) when used with typical pediatric-sized catheters.