This study was undertaken to determine the effect on blood gas determinations of an incomplete purging of the heparinized flush solution from an indwelling arterial catheter and pressure tubing. Arterial blood gases were measured serially after withdrawing 2,4,6,8,10, and 12 ml of flush-blood solution from a 20-gauge radial artery catheter which was connected to one of two kinds of pressure tubing (4-ft and 6-ft Cobe pressure lines). In those samples from the 4-ft Cobe pressure tubing the pH was nearly unchanged in samples 2 thru 6, while the PaCO₂, PaO₂, actual bicarbonate, and base excess remained approximately constant in samples 3 thru 6. The results of samples taken from the 6-ft tubing were that the pH remained unchanged from samples 3 to 6, and PaCO₂, actual bicarbonate, and base excess remained the same from samples 4 onward. PaO₂ was unchanged in all sample. In conclusion we suggest that at least 4 ml of blood from a 4-ft Cobe pressure tubing and 6 ml from a 6-ft tubing should be withdrawn prior to arterial blood gas measurements.