The imprecision arising from the application of the Henderson-Hasselbalch relationship to the blood of anesthetized dogs.

1. Arterial blood samples were obtained from anaesthetized dogs in which either a respiratory alkalaemia, a respiratory acidaemia or a non-respiratory acidaemia was produced and from animals in which no deliberate changes in acid-base balance were produced. The samples were analysed for pH, carbon dioxide tension and concentration, oxygen tension and concentration and haematocrit. From these determinations the plasma carbon dioxide concentration, the plasma bicarbonate ion concentration and pK' were calculated.2. The value of pK' lay within the range 5.86-6.27. The temperatures of the animals studied were between 36.75 and 39.25 degrees C but there was no significant relationship between pK' and temperature.3. The values of pK' increased with an alkalaemia and decreased with an acidaemia.4. In addition to the variation of pK' with pH there was a considerable residual scatter of pK' values at any one pH value.5. It was concluded that in these experimental conditions the use of the Henderson-Hasselbalch equation in a form not dealing with activities was unacceptable and could lead to much larger errors than has hitherto been thought.