1. The projections from one of the paths (b-VF-SOCP) in the ventral spino-olivocerebellar system to the cortical b-zone located in the lateral part of the anterior lobe vermis and to the lateral vestibular nucleus (LVN) have been studied in cats with the spinal cord transected at C3 sparing only the contralateral ventral funiculus. The projection to the b-zone was studied by recording climbing fiber responses in single Purkinje cells on stimulation of limb nerves. The projections to the LVN, direct through climbing fiber collaterals and indirect through Purkinje cells, were studied by recordings EPSPs and IPSPs in LVN neurons. 2. The Purkinje cells in the b-zone were arbitrarily divided into five groups with different inputs and occupying different microzones each with a width of about 200 micron. On passing medially across the b-zone the microzones had the following input characteristics: 1. activation exclusively from hindlimb nerves, 2. short-latency activation from hindlimb and long-latency activation from forelimb nerves, 3. short-latency activation from hindlimb and forelimb nerves, 4. short-latency activation from forelimb and long-latency activation from hindlimb nerves, and 5. activation exclusively from forelimb nerves. 3. The five microzones projected to different groups of LVN neurons which occurred intermingled throughout the nucleus. The LVN neurons inhibited from a certain microzone were activated by the collaterals of the climbing fibers projecting to that microzone. 4. The organization of the spino-olivo-cerebello-vestibulo-spinal path is discussed. It is suggested that the microzone and collection of subcortical neurons represent the basic computational unit of the cerebellum.