1. The medulla of the tench brain was searched systematically by means of needle electrodes for rhythmic bursts of action potential activity coinciding with the breathing movements.
2. The neurones which produced these rhythmic bursts of activity were located in the grey matter, mainly beneath the IXth and Xth motor nuclei and in the region round the VIIth motor nucleus. This type of activity was also found in some of the neurones forming the Vth and VIIth motor nuclei.
3. The respiratory neurones were not arranged in a discrete and homogenous nucleus anywhere in the medulla, but were scattered through the grey matter. The distribution was not uniform, the neurones tending to occur in very small groups. There was also a relatively higher density of respiratory neurones in the central, as compared with the more anterior and posterior, parts of the respiratory region. The possibility that variations may occur in the constitution of the respiratory centre, in different individuals and in the same individual at different times, is considered.
4. The manner in which neurones of the respiratory centre function to produce the rhythmic activity is discussed. Localized destruction of active respiratory regions, over a wide area of the medulla in different fish, was never followed by a breakdown in the rhythmic movements. This is interpreted as evidence against the existence of a pacemaker and favouring the hypothesis that the rhythm is produced by a general reciprocal interaction of large numbers of respiratory neurones.
