1. Spike-triggered averaging was used to reveal focal synaptic potentials (FSPs) in the thoracic ventral horn resulting from impulses in individual respiratory interneurones situated in the contralateral ventral horn of the same segment (T6 or T7) in anaesthetized paralysed cats. 2. FSPs were of two types, negative-going or positive-going, and were of the same sign at each of several sites for each unit. These two types were interpreted as indicating excitatory or inhibitory actions respectively. 3. The positive-going FSPs were almost all the result of activity in the most strongly modulated, phasic respiratory interneurones, whereas most of the negative-going ones were derived from neurones with a continuous tonic component in their firing patterns. 4. Positive-going FSPs (particularly those from inspiratory neurones) were generally found at more ventral sites in the ventral horn than negative-going ones. 5. It is suggested that an important role for the phasically active interneurones is to provide phasic inhibition, which is combined with largely tonic excitation to produce strong respiratory modulation in the activity of other neurones in the thoracic spinal cord, particularly in motoneurones. However, some groups of respiratory interneurones, such as a group of tonic inspiratory ones which are located more dorsally than the phasic inspiratory ones, may not receive such phasic inhibition.