The DNA metabolism of mouse spermatogenic cells was investigated by intravenous administration of isotope and Staput fractionation of the cells. The pattern of metabolism is virtually identical with that observed in Lilium microsporocytes. A programmed single strand nicking of DNA occure at pachytene such that at least 50% of the DNA is in the form of 62S fragments. Repair replication of endogenously nicked sites is fully achieved during pachytene. The sites of nicking and repair are preferentially located in sequences that are repeated about 400 times. These results are considered as strong evidence for a universal pattern of meiotic prophase DNA metabolism which is associated with crossing-over.