It was shown using the micronucleus test and estimating the defects of sperm heads that premeiotic and meiotic mammalian cells are genetically very sensitive to Dipin and nitrosomethylurea. In rats, unlike mice, the stem and differentiating spermatogonia with serious chromosomal defects are not eliminated and pass through a "sieve" of mitotic and meiotic divisions reaching the stage of round spermatids. Our observations suggest long-term preservation Dipin-induced, rather than nitrosomethylurea-induced, mutations in the stem cells of both mice and rats.