Electrophysiological studies on the cerebellothalamocortical projections of cats and monkeys are presented and discussed, referring mainly to the mechanisms of thalamocortical (TC) projections. Two types of TC projections, deep and superficial, were distinguished electrophysiologically. The former corresponds mostly to the 'specific' TC projection system which can be traced with conventional histological methods, whereas the latter constitutes the final direct TC pathway of the recruiting systme ('nonspecific' or 'diffuse' TC projection system). Superficial TC neurones were identified electrophysiologically in the thalamus and their distinctive responses were demonstrated in relation to cortical recruiting responses, spindling waves and desynchronized activation of the cortex due to the midbrain reticular formation. The characteristics of superficial TC projections and their cortical distributions appear to indicate that the superficial TC projections are much more developed in primates, probably most in the human, and that they convey the impulses responsible for more associative and elaborate integrations in the cerebral cortex.