The interstitial nucleus of Cajal (INC) is a mesencephalic nucleus considered to be an important premotor centre for eliciting vertical and rotatory eye and head movements. An ultrastructural analysis was carried out on the cat INC to elucidate the structure of the neurons and neuropil and, in particular, to examine the organisation of the synaptic relationships. The neuronal population is formed by a substratum of small and medium-sized neurons and by a limited number of larger neurons scattered throughout the nucleus. The small and medium-sized neurons display ultrastructural features similar to those shown by the cells of the central reticular formation. The larger neurons differ from those of the rest of the INC both in dimensions of their somata and in certain cytological characteristics (e.g. a well developed rough endoplasmic reticulum). Synaptic boutons are scarce at the level of the nerve somata and very abundant in the neuropil, thus indicating the latter as the main target field for the afferent inputs to the INC. Again the larger neurons are exceptions as their cell surface is extensively invested with axon endings. It is suggested that the larger neurons should be considered as a distinct cell population within the nucleus. On the basis of a morphometric analysis carried out on the synapses it is possible that in the INC, at the level of the somatic synapses, a mainly inhibitory role may be exerted, whereas in the neuropil it is possible that there is also excitatory control and modulation.