1. Unitary EPSPs were recorded intracellularly from neurons of the red nucleus (RN) by stimulating the cortico-rubral fibers in normal cats as well as those with chronic lesion of interpositus nucleus (IP) of the cerebellum. 2. Two groups of unitary EPSPs were recorded in cats with IP lesion. One consisted of cortico-rubral unitary EPSPs with a shorter time to peak and larger amplitude than those in normal cats. The other consisted of unitary EPSPs of the normal range. 3. The appearance of the fast rising group of cortico-rubral EPSPs caused by IP lesions was theoretically predictable based on Rall's compartmental model by assuming that new synapses were formed close to the soma of the RN cells. 4. The amplitude of the fast rising EPSPs increased more prominently by hyperpolarizing the membrane potential than that of the slowly rising ones. 5. It was concluded that the fast rising group of unitary EPSPs which appeared following IP lesion was due to the formation of new synapses on the proximal portion of the soma-dendritic membrane of RN cell.