Genetic analysis of learning and memory deficits (LMD) in senescence-accelerated mouse P8 (SAMP8) was performed by cross-mating SAMP8 and Japanese Fancy Mouse 1 (JF1). The incidence of LMD in the F2 generation showed a 3:1 segregation ratio of mice with LMD to normal mice, and the incidence of LMD in the backcross generation of the F1 to JF1 parental strain was in agreement with a 1:1 ratio of mice with LMD to normal mice. Estimation of the number of genes involved in the development of LMD using Wright's formula showed that at least two to four genes are involved. These results suggest that the inheritance of LMD is polygenically controlled and that there may be a single major gene, but this locus is not sex-linked. Moreover, hormonal influence on the development of LMD in SAMP8 females is of a genotype-dependent manner.