In the present study, stereological techniques applied to electron micrographs of the molecular layer of the rat cerebellum have been used to estimate the number of parallel fiber synapses on the dendritic tree of a single Purkinje cell. Quantitative features of the parallel fiber to Purkinje cell dendritic spine synapses and of the parallel fibers were investigated as a preliminary to estimating the number of synapses. Parallel fiber to Purkinje cell synapses are flattened disclike structures with a mean axial ratio of 1.47 and a mean diameter of 319 μm in fixed tissue. The density of synapses in our fixed material was 8.17 × 108 per μl of molecular layer. Determination of the length density of the synapses per unit area of micrograph indicated a synapse density of 8.03 × 108 per μl. These densities give a total number of synapses per Purkinje cell of 1.74 × 105 and 1.71 × 105, respectively. Estimation of the number of parallel fiber varicosities and of varicosity length gave a density of 9.31 × 108 varicosities per μl of molecular layer and determining the mean number of parallel fiber to Purkinje cell synapses per varicosity gave a synapse density of 9.82 × 108 per μl, equivalent to 2.09 × 105 per Purkinje cell. The reasons why this estimate is likely to be too high are discussed. We conclude that there are some 175,000 parallel fiber synapses on an individual Purkinje cell dendritic tree in the cerebellar cortex of the rat, considerably more than previously reported.