In 34 cats, the changes in extracellular potassium ion activity (aK) and extracellular spike activity within the pool of respiratory neurones in the dorsormedial and ventrolateral medulla were recorded using microelectrodes filled with a liquid potassium ion exchange resin. Cyclic changes in aK which parallel central respiratory activity were restricted to those regions where respiratory neurones are known to be localized. The largest changes in aK (0.1--0.3 mmol . 1(-1)) were found within the ventral pool of inspiratory neurones. The aK increased during inspiration in parallel with the pattern of phrenic nerve activity. The smallest changes in aK (0.02--0.06 mmol . 1(-1)) were observed within the ventral pool of expiratory neurones. Here, aK showed a transient increase during both inspiration and expiration. Within the dorsal pool of inspiratory neurones, small fluctuations of aK were observed paralleling phrenic nerve activity and the afferent discharge of the intact vagal nerves. After the vagal nerves were cut, the changes in aK then paralleled phrenic nerve activity. The variations in aK within the ventral pool of respiratory neurones did not change after bilateral section of vagal nerves. Repetitive stimulation of the vagal nerves (0.1--0.5 V, 0.05 ms) produced an increase in aK only within the dorsal pool of inspiratory neurones, whereas repetitive spinal cord stimulation (5--10 V, 0.05 ms) resulted in an increase of aK within the ventral pool of respiratory neurones. The amplitude of the cyclic changes in aK increased significantly whenever the electrode approached individual respiratory neurones as verified by the amplitude and shape of the spikes recorded by the reference barrel. The maximal changes in aK then reached a peak amplitude of 1.3--1.5 mmol . 1(-1), the pattern of aK changes resembling that measured within the pools of neurones. The aK started to rise prior to the discharge of action potentials, indicating that the efflux of K + -ions was produced as a consequence of synaptic transmission. The functional importance of these changes in extracellular potassium is discussed.