We studied the dynamics of age-related cytogenetic changes in the developing male germ cells of mice prone to accelerated senescence (strain SAMP1) and mice resistant to accelerated senescence (strain SAMR1) by counting the spermatogonial and meiotic micronuclei and testing the defects of the spermatozoon head shape. In these animals, the accumulation of germinative mutations has a nonlinear pattern. During the entire ontogenesis of SAMP1 mice, the frequency of circular spermatids with micronuclei corresponded to the level observed upon induced mutagenesis.