Following injections of horseradish peroxidase into the cerebellum, the distribution of labeled neurons was studied in the whole length of the spinal cord of the rat. To find the ascending side of the axons, injections were made following hemisections at C1 or between C1 and C2. Labeled spinocerebellar tract neurons were classified into two groups according to the axonal course in the spinal cord; one is composed of neurons with uncrossed ascending axons and the other, neurons with crossed ascending axons. Neurons of origin of the uncrossed tracts were located in the medial part of lamina VI of C2 to C8, the central part of lamina VII of C4 to C8, lamina V of C7 to L3 and Clarke's column. Neurons of origin of the crossed tracts were found in the central cervical nucleus of C1 to C3, the intermediate zone and the ventral horn of the lower thoracic and the lumbar segments (T11 to L3), and in the dorsal horn, the medial part of lamina VII and the ventrolateral part of the ventral horn of the sacral and caudal spinal cord. In comparison with our previous results in the cat, it was suggested that the spinocerebellar system in the rat is organized in the same fashion as in the cat, in terms of the location and the intraspinal axonal course of the cells of origin.