The establishment of the adult innervation of Purkinje cells (PCs) by climbing fibers (CFs) was studied in the cerebellar vermis of the developing rat. Excitatory postsynaptic potentials (EPSPs) evoked in PCs by activation of the climbing fibers (CF-EPSPs) were recorded intracellularly from a total of 310 cells in young rats aged from 3 to 15 postnatal days. The CF system was activated by electrical stimulation of either the inferior olive (IO) nucleus or the region near the fastigial nucleus (juxtafastigial or JF stimulation). A given PC at each age was considered to be innervated by more than one CF when the amplitude of the spontaneous or evoked CF-EPSPs fluctuated in a stepwise manner. On the other hand, innervation of a PC by a single CF was established on the basis of the all-or-none character of CF-EPSPs. Two parameters were followed throughout development, the percentage of multiply innervated PCs and the mean number of steps in the evoked CF- EPSPs. The data presented confirm the transient multiple innervation of PCs by CFS on postnatal days 8 and 9 (Crepel, F., J. Mariani, and N. Delhaye-Bouchaud (1976) J. Neurobiol. 7: 567–578) and strongly suggest its existence at earlier stages (from postnatal day 3). Moreover, it is shown that the multiple innervation was maximal on postnatal day 5 and then decreased until the innervation by a single CF was established on day 15.