Meiosis in the male mouse has been studied autoradiographically in air-dried preparations. Information has been obtained on the relative rates of DNA synthesis and the lengths of the S-periods in spermatogonia and spermatocytes. The average rate of synthesis in the spermatocyte is lower, and the S-period is of longer duration than the preceding spermatogonial generations. The labelling pattern of the sex-chromosomes and autosomes observed at diakinesis and metaphase II in cells labelled at the spermatocyte S-period appears to be similar to that found in cells labelled during the spermatogonial S-periods. Replication in the autosomes commences before the sex-chromosomes. Late replicating autosomal centric regions show a marked degree of asynchrony in labelling both between and within bivalents. The Y chromosome starts and finishes replication later than the X. There is a short, late-replicating, segment of the X in the vicinity of the centromere. There is a short, early-replicating segment of the Y in the vicinity of the centromere which may represent the euchromatic short arm. The X and Y appear to associate at diakinesis by the distal ends of their long arms.