During external cardiac massage and after restoration of spontaneous circulation, the arterial and central venous blood gas status of ten patients was determined. During cardiopulmonary resuscitation the median arterial pH value was 7.29 and the median central-venous pH value was 7.16. The low central-venous pH during resuscitation was probably caused by the high partial pressure of carbon dioxide, because no significant difference between arterial and central-venous base deficit was found. The arteriovenous pH and carbon dioxide gradients were significantly lower after spontaneous circulation had been restored. The arterial pH does not parallel the marked fall in central venous pH, and therefore only partly indicates acid-base changes during resuscitation. On the other hand, a central-venous blood gas status not only indicates the degree of metabolic acidosis present, but also the "respiratory" acidosis that in turn is a measure of the severity of intracellular acidosis.