1. A model system of the thermal dilution technique was used to determine in vitro the loss of thermal indicator across different sizes of polyethylene catheters. 2. Thermal indicator loss (caloric gain by the indicator fluid or caloric loss by the catheter) per unit length of catheters (H/L) increased as a function of the outer radius (OR) of the catheters. Linear regression analysis gave the following equation: H/L=0.337OR+0.073 (r=0.88, P<0.05). 3. Thermal indicator loss per unit surface area (H/SA) decreased almost linearly with the following regression equation: H/SA=-1.860 OR+0.364 (r=0.97, P<0.97, P<0.01). 4. Thermal indicator loss per unit volume contained in the catheter (H/vol) decreased exponentially according to the following equation: H/vol =89.le-25.4(OR) (r=0.97, P<0.01). 5. This study indicates that there is a measurable loss of thermal indicator during injection corresponding to a loss between 9 and 16% (for a 11.5 cm length of PE 50 and PE 240) of the required calories for warming the injectate to 37 degrees C. Data is provided for correction of heat loss across the wall of a given length and diameter of polyethylene tubing.