The mean arterial blood PO2 values of 20 patients' samples taken into Sarstedt 2 mL Monovette syringes were found to be initially 0.43 kPa higher and after 15 min storage 0.93 kPa higher than the mean values using Corning 3 mL Blood Gas Kit syringes. The Sarstedt syringe allowed entry of small air bubbles and the plunger was noted to be stiff. We speculated that high injection pressures might increase measured PO2. Using each type of syringe, 20 ITU staff injected saline into a Corning 178 pH/blood analyser linked to a pressure transducer via a three-way connector. Mean injection pressures for Sarstedt and Corning syringes were 17.15 and 6.42 kPa, respectively (P less than 0.005). Following injection at pressures between 2.8 and 26.6 kPa, the mean PO2 of equilibrated saline containing an air bubble was 0.80 kPa higher than the mean value obtained at injection pressures of less than 2.8 kPa. However, with no air bubble the mean PO2 of equilibrated saline was still 0.45 kPa higher when injected at pressures between 2.8 and 26.6 kPa than the mean value obtained with injection pressures of 2.8 kPa. We suggest that for optimal PO2 determinations syringes should not only allow minimal air contamination but also have plungers that reduce injection pressure to a minimum.