Cerebellar Purkinje cells receive two distinctive types of excitatory inputs. Climbing fiber (CF) synapses have a high probability of release and show paired-pulse depression (PPD), whereas parallel fiber (PF) synapses facilitate and have a low probability of release. We examined both types of synapses using serial electron microscopic reconstructions in 15-d-old rats to look for anatomical correlates of these differences. PF and CF synapses were distinguishable by their overall ultrastructural organization. There were differences between PF and CF synapses in how many release sites were within 1 microm of a mitochondrion (67 vs 84%) and in the degree of astrocytic ensheathment (67 vs 94%). However, the postsynaptic density sizes for both types of synapses were similar (0.13-0.14 microm(2)). For both types of synapses, we counted the number of docked vesicles per release site to test whether this number determines the probability of release and synaptic plasticity. PF and CF synapses had the same number of anatomically docked vesicles (7-8). The number of docked vesicles at the CF does not support a simple model of PPD in which release of a single vesicle during the first pulse depletes the anatomically docked vesicle pool at a synapse. Alternatively, only a fraction of anatomically docked vesicles may be release ready, or PPD could result from multivesicular release at each site. Similarities in the number of docked vesicles for PF and CF synapses indicate that differences in probability of release are unrelated to the number of anatomically docked vesicles at these synapses.