In vivo methylation of DNA in different organs of rat at various ages.

W e investigated in vivo labelling of D N A in single organs of female Wistar rats after application of 3H-methionine. In 40 g and 120 g rats the tritium incorporation into D N A was 10 times higher in spleen than in liver, kidney and heart. D N A methylation in newborn rats (6 — 8 g) was more pronounced in each of these organs. W e demonstrated a varying degree of tritium in corporation into the different bases of D N A depending on the organs. W e claim that only the values for 5-methylcytosine should be used as sole criteria of D N A methylation. Our results indi cate that D N A methylation in vivo is enhanced in those organs, where a more intensive D N A synthesis occurs.