Patterns of DNA replication, fluorescence, and meiotic pairing have been studied in the composite X and Y chromosomes of a rodent, the soft-furred field rat, Millardia meltada. The heterochromatin of both the chromosomes replicates during late S and fluoresces brightly with Netropsin-Olivomycin. The fluorescence with Actinomycin D-Hoechst is dull, suggesting that the heterochromatin of both the X and the Y is relatively GC-rich. When surface-spread testis cells are analysed after silver staining considerable portions of the X and Y exhibit synapsis. On the basis of this study it appears that the heterochromatin of the X chromosome of M. meltada is substantially similar to that of the Y chromosome.