In contrast to the current concept of abortive regeneration of mammalian central axons, the occurrence of marked, functionally active, regeneration of the cerebellofugal projection was proved in the cat after complete transection of the decussation of the brachium conjunctivum (BCX). Because the BCX is a complete crossing it was transected completely by pushing down an edged U-shaped wire to the base of the brain stem in the midline, and the wire was left in situ to mark the lesion. Later, horseradish peroxidase was injected into the cerebellar lateral and interpositus nuclei to label the cerebellofugal projection arising from these nuclei; axonal regeneration was proved by demonstration of labelled fibres passing through the area enclosed by the U-shaped wire. By this procedure the origin, course, and destination of the regenerated fibres were identified unambiguously. Most of the regenerated axons took a course similar to that of the normal projection and terminated in the normal projection areas, whereas a small proportion of fibres showed an aberrant course and termination. Functional connectivity of the regenerated cerebellothalamic projection was tested electrophysiologically in the same animals examined morphologically. In all animals in which marked axonal regeneration occurred, cerebellocerebral responses, as in intact animals, were evoked in the frontal motor and parietal associated cortices. Study of the time-course of regeneration revealed that the cut ends of axons began to swell as early as 15 min after transection, produced terminals tipped by growth cones in 14-24 hours, grew to cross the lesion in 3 days, and distributed dense terminals in the thalamus by 19 days.