In vivo BrdU-dye methodology is used to reveal patterns of replication kinetics and meiotic recombination in male germ cells of the Armenian hamster. Analogue substitution over a fraction of the DNA synthesis period results in distinctive 33258 Hoechst fluorescence staining of late replicating chromatin, detectable at spermatogonial or spermatocyte I and II stages. Spermatogonial cells which are extensively substituted with BrdU over the penultimate synthesis period reveal sister chromatid differentiation in all chromosomes of primary or secondary spermatocytes. In chiasmatic regions, exchanges between unlike-stained non-sister chromatids are indicated by isolatelled segments, while those occurring between like-stained non-sister chromatids are not directly detected. In sex chromosomes from young animals, these alternative images occur in a ratio of 1:1, which supports the concept that homologue non-sister chromatid regions are randomly broken and re-united in the process of chiasma formation. Deviations from randomness appeared to occur in older animals. Sex bivalent chiasmata are either coincident with points of visible exchange, or they appear to have variable degrees of terminalization. Secondary spermatocytes display sharp chromatid contrast, and aid in mapping the positions and frequencies of homologue exchanges.