Right ventricular (RV) ejection fractions have been difficult to estimate clinically. It has been demonstrated recently that RV ejection fractions can be calculated by thermodilution techniques using a rapid response thermistor and computer. This method critically depends on adequate mixing of the thermal bolus and sensing of the rapid response thermistor. This study examined the effects of the thermistor position within the pulmonary artery and injectate site within the right atrium on RV thermodilution ejection fraction measurements. Ten pigs were instrumented with a RV thermodilution catheter in the pulmonary artery, an injectate catheter in the right atrium, an atrial-pacing electrode, and a systemic arterial catheter. The RV ejection fractions were determined using thermodilution in two ways: (1) with incremental increases in pulmonic valve to thermistor distance, and (2) with incremental increases in injectate port to tricuspid valve. These measurements were obtained at a paced rate of 107 +/- 1 beats per minute (bpm) and then repeated with pacing-induced tachycardia (140 bpm). The highest RV ejection fraction with the lowest coefficient of variation was with the thermistor 2 cm from the pulmonic valve (50 +/- 2 percent), with a significant decline from this value at 10 cm (42 +/- 4 percent, p less than 0.05). This reduction in RV ejection fraction values with increased pulmonic valve to thermistor distance became more pronounced with tachycardia where a significant decline in RV ejection fraction occurred at 4 cm from the valve when compared with 0 cm (38 +/- 6 percent vs 47 +/- 3 percent, respectively, p less than 0.05). There was no significant change in RV ejection fraction at any injectate port to tricuspid valve distance at the lower heart rate. With tachycardia, however, a significant decline in RV ejection fraction occurred with the injectate port located 7 cm from the tricuspid valve (p less than 0.05). These results demonstrate that RV ejection fractions can be reliably obtained using thermodilution. Positioning of the thermodilution catheter is an important consideration for obtaining optimal RV ejection fraction measurements. Care should be taken to position the catheter with the thermistor a minimal distance from the pulmonic valve and the injectate port within the central body of the right atrium.