An analysis was made of the effect of sodium heparin solution on the determination of arterial blood PCO2, pH, and bicarbonate. Dilution of blood by heparin solution did not affect the pH of blood samples significantly but reduced the measured PCO2 and th calculated bicarbonate and base excess in direct proportion to the amount of dilution. Used in the usual quantities as an anticoagulant, lubricant, and filler of needle and syringe dead space, heparin solution caused a 17 per cent or greater reduction in PCO2 and bicarbonate in approximately 17 per cent of the arterial blood samples currently received for analysis in a university hospital laboratory. Such unrecognized errors could cause misinterpretations of acid-base status. Completely filling the sampling syringe with blood and using a small-gauge needle will reduce the error.