In recent years, wide DNA changes have been described among and within plant species. As to intraspecific DNA variations, they have been reported in a number of species, namely, Linum usitatissimum, Microseris douglassi, M. bigelovii, Zea mays, Helianthus annuus and others. DNA changes have been detected mostly by Feulgen cytophometry, that, if experiments follow some general rules, may be adequate for quantitative analyses. Biochemical analyses have, in some cases, confirmed the cytophotometric findings; it is generally accepted that DNA variations involve loss or increase of repeated DNA sequences (including rDNA). Concerning the origin and inheritance of DNA changes, a marked deviation from normal segregation rates are found in different species, and genome size seems to change following precise mechanisms. The functional significance of DNA changes is discussed: in many (but not all) cases a relationship has been established between DNA changes and adaptation to different environments, through nucleotypic effects. These aspects are summarized with particular reference to intraspecific DNA variations in the sunflower, which occur in cultivated varieties, hybrids and inbred lines within a single generation.