Interest in the acid base metabolism in mental disease has existed almost as long as the relevant estimations have been available to clinicians. In the period from 1920 to 1938 many careful and accurate determinations of the factors involved were carried out, both in Europe and the United States of America. The results, while being sometimes equivocal, for the most part showed that there was no demonstrable change between those suffering from schizophrenia or depression and a normal hospital population. Since 1938 little interest appears to have been taken in the hydrogen ion concentration of the blood and the pCO2 in mental disease. During recent years great progress has been made in the apparatus for the determination of blood pH and pCO2 and fairly recently it became possible to estimate the first of these three places of decimals with a reproducibility of 0·005 of a pH unit and an equivalent accuracy for pCO2. In view of this great improvement in the apparatus available, it seemed worth while to estimate the blood pH and pCO2 in a large number of chronic mentally ill patients. The facilities and the patients were available in Hollymoor Hospital, so we decided to see whether with the much more delicate instruments now available it was possible to demonstrate any significant difference in the blood pH and pCO2 between schizophrenics and other mentally ill patients.