1. Injections of horseradish peroxidase (HRP) suspension were made in the cerebellar cortex of cats (most often the paramedian lobule). The staining of the cerebellar cortex and the ensuing labeling of neurons in the inferior olive were studied in experiments with variations of concentration of HRP, amounts of fluid injected, survival time and age of the animals. Light microscopical studies were supplemented with electron microscopical observations. The folded cerebellar cortex offers particular difficulties with regard to obtaining a predictable extent of stained tissue, and the spreading of the fluid within the cortex shows great variations even with the same amounts and concentrations of HRP suspension. Diffusion of fluid appears to occur most easily within the molecular layer. Often there are unstained parts of folia between stained parts. Staining of the cortex is barely visible after 7 days, but appreciable shrinking of the stained area does not appear to occur until after 4 days. 2. The first signs of labeling of olivary neurons are seen after 5-10 hours, after 7 days there are no labeled cells. The rate of retrograde transport in olivocerebellar fibers is calculated to be between 50 and 100 mm/day. Labeling of cells appears to require staining of the molecular layer of its projection areas in the cerebellum. 3. For studies of the olivocerebellar projection survival times of 2-3 days and injections of 0.5 mul of a 50% HRP suspension seem in general to be well suited. Best results are obtained with animals weighing 1-3 kg. There is a clearcut correlation between the site of staining of the cortex of a particular part of the cerebellum and the site(s) and extension of olivary area(s) containing labeled cells. 4. Anterograde transport in axons of Purkinje cells has been observed. Electron microscopically the axons of these fibers contain HRP labeled tubules and vesicles as do their terminal boutons in the nuclei. 5. In cases where the injected fluid has spread to the cerebellar nuclei, localized parts contain neurons which are labeled as are the cells in the injected cerebellar cortex.