DNA synthesis is thought to be confined to a specific segment of interphase (S) preceding cell division and to be completed prior to the onset of either mitosis or meiosis1. Chromosome pairing and crossing over between homologous chromosomes in meiosis are separate events, both of which involve the DNA moiety of the chromosomes. An explanation of these processes in terms of DNA synthesis has only recently been suggested2,3. Hotta et al.4–6 have demonstrated by biochemical techniques that approximately 0.3 per cent of the total DNA complement of the cell is synthesized in the zygonema-pachynema stages of meiotic prophase in lily microsporocytes. We have cytological evidence of a little DNA synthesis during early pachytene stage in the male mouse.