The fate of the multiple innervation of Purkinje cells (PCs) by climbing fibers (CFs) was studied as a function of age in immature rats rendered agranular by X-irradiation, in immature hypothyroid rats, and compared to that in controls. This was done by examining in each group the intracellular activities of PCs mediated via CFs throughout maturation. From the third day in control rats, CF responses of PCs evoked by juxta fastigial region (JF) stimulation or occurring spontaneously already resembled the adult responses with, however, some important differences: (1) most of these responses were graded by steps with the intensity of the stimulation before day 13, due to the multiple innervation of PCs by CFs (see below); (2) immature CF responses exhibited a longer duration and their initial spike started near the peak of the EPSP instead of near the baseline later on. Finally, an anlage of CF response was already present in most PCs on day 2, and consisted of a single fast spike elicited near the peak of an underlying all-or-none EPSP. In the 3 groups of rats, CF EPSPs already closely resembled the adult ones as early as 3 days, although their total duration and especially their time to peak were longer. In control rats, these CF EPSPs reversed with depolarizing currents from day 3 and currents for reversal were much lower than in the adult. 'Dual' CF EPSPs of PCS37 were encountered in immature 7- to 10-day-old controls, and persisted in hypothyroid rats until the end of the third postnatal week. The mono- or the multiple innervation of PCs by CFs was ascertained in th 3 groups according to the graded or the all-or-none character of CF EPSPs, and the number of CFs impinging on a given PC was estimated by the number of steps in the response. In control rats, most of PCs were already multiply innervated by CFs as early as 3 days. The multiple innervation culminated on day 5 with an average number of 3.4 CFs for PC, and rapidly regressed later on, so that the adult-type monoinnervation was the rule after day 13. In hypothyroid rats, the establishment of the redundancy and its regression was delayed by 2--3 days. In X-irradiated rats, the settlement and the involution of the multiple innervation of PCs by CFs was exactly superimposed with that seen in controls until day 8. Later on, regression of the supernumerary contacts no longer occurred so that most PCs remained multiply innervated until adulthood. Finally, the first clear-cut IPSPs were detected in PCs on day 9 in control and X-irradiated rats and 2--3 days later in hypothyroid animals.