We have refined the map positions and identified molecular markers for three neurological mutations in the mouse, tottering (tg), Purkinje cell degeneration (pcd), and nervous (nr). These mutations were localized using simple sequence length polymorphisms between the mouse strain on which the mutation arose and the inbred strain onto which the mutation was bred. This approach to mutation mapping is generalizable to any mutant that has been backcrossed for several generations. The tg mutation was localized to the 1.1 cM region of chromosome 8 distal to simple sequence repeat (SSR) D8Mit103 and proximal to SSRs D8Mit79, D8Mit105, and D8Mit283. The pcd locus was mapped to the 5 cM interval of chromosome 13 between SSRs D13Mit139 and D13Mit67, and the nr locus was mapped between SSRs D8Mit155 and D8Mit18, a 5.6 cM region of chromosome 8. For each mutation, several SSRs distinguishing mutant from wild type chromosomes were identified within these regions. The definition of molecular markers distinguishing mutant from wild type alleles makes possible for the first time identification of tg, pcd, and nr mutants prior to behavioral manifestation of the mutant genotype. Thus, developmental studies of these mutants designed to describe or dissect the biochemical basis of the induction of the mutant phenotype are now feasible.