The intramembrane particle (IMP) content of Purkinje, basket, stellate and Golgi cell plasma membrane was quantitatively assessed in freeze-fracture replicas of the cerebellum of normal rats and Weaver mutant mice. This analysis showed that, irrespective of the cell type innervated (i.e. Purkinje, stellate, basket or Golgi cells) postsynaptic membranes for parallel fibers had a relatively low IMP content in their cytoplasmic P-face (approximately equal to 750 IMP/micron2), while postsynaptic membranes for climbing, basket and stellate axons were characterized by a significantly higher IMP content (approximately equal to 1400 IMP/micron2). This difference of IMP content between the targets for parallel fibers and those for climbing, basket and stellate axons was restricted to IMP smaller than 10 nm and appeared progressively during the development of the molecular layer, suggesting a correlation between the formation of synaptic contacts and the segregation of the postsynaptic membrane in these two different domains. In addition, the study of the Weaver mice cerebellum, which is deprived of parallel fibers, but yet shows a normal IMP content in the postsynaptic membrane for the missing fibers, indicated that this characteristic IMP content is established before or during the afferent's reaching its target, and independently of whether the contact ultimately occurs.