1. To assess the non-respiratory acid—base disturbance which results from cardiorespiratory arrest experiments were performed on anaesthetized dogs, and circulatory arrest was established either by inducing ventricular fibrillation or by complete occlusion of the great veins.

2. In the first series of experiments (group A) the period of cardiorespiratory arrest was limited to 2·0–3·5 min. It was shown that the non-respiratory component of the ensuing acid—base disturbance as expressed by the non-respiratory pH was small: in nine episodes of circulatory arrest after occlusion of the great veins there was a mean reduction in the non-respiratory pH of 0·04 pH unit (range 0·025–0·055), and in seven episodes of ventricular fibrillation the mean reduction was 0·024 pH unit (range 0–0·055).

3. In the second series of experiments (group B) the period of cardiorespiratory arrest was varied from 3·0 to 11·0 min. It was found that the degree of non-respiratory acidaemia was correlated with the duration of arrest but the magnitude of these changes was not large.

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