Experiments are described which emphasize the importance of avoiding loss of carbon dioxide when estimating the pH or bicarbonate concentration of ruminal fluid. The high pCO2 of ruminal fluid is stressed; this may be 10 times or more as great as that of arterial blood. The relationship between pCO2, pH, and [HCO3-] was examined in terms of the Henderson-Hasselbalch equation over a wide range of pCO2. From this, the pK1' of the carbonic acid system in four ruminal fluids was determined as 6.21-6.28, mean 6.25. The higher pH of saliva-free samples of ruminal fluid withdrawn by suction through a tube passed down the oesophagus, as compared with that of the bulk fluid obtained through a ruminal fistula, is considered to be due to loss of carbon dioxide during collection. A better estimate of intraruminal pH is obtained, even when salivary contamination occurs, if such samples are equilibrated with a sample of the animal's ruminal gas; if this is not practicable, an arbitrary gas mixture of high pCO2, e.g. 50 per cent. carbon dioxide and nitrogen, may be used.