Multiple innervation of cerebellar Purkinje cells (PCs) by climbing fibers (CFs) has been described recently in adult weaver, reeler, and staggerer mutant mice, instead of the monoinnervation found in normal adults. In the present study, the extent of this multiple innervation was estimated by two methods, using both evoked and spontaneous activity of the olivocerebellar system. Concordant values were obtained: the mean number of CF collaterals per PC was between 3.5 and 4 in weaver and staggerer and close to 3.2 for the multiply innervated PCs of reeler mice. These values are of the same order of magnitude as those for the transient multiple innervation in developing rats (Mariani and Changeux, 1981a,b).