The discovery of the phenomenon of nonhomologous pairing in the female of Drosophila melanogaster provided a genetic tool for analyzing chromosome behavior, which has made it possible to establish a temporal order of meiotic events that includes two types of pairing, one preceding exchange (exchange pairing) and one following exchange (distributive pairing). Studies with free X-duplications support the assumption that the initial pairing for exchange involves parasynapsis rather than short, effectively paired regions. Recognition at exchange pairing is correlated with the extent of euchromatic homology in the duplication up to a physical length between 11 μ and 25 μ of the salivary gland chromosome, at which point full recognition (equivalent to the same length of homology carried on a normal X chromosome) is achieved. Recognition at distributive pairing is correlated with total size of the chromosome, is independent of homology, and is restricted to chromosomes which have not undergone exchange with an independent homologue. Temperature-treatment and labeling studies indicate that there is a rough coincidence between the temperature-sensitive period for recombination and that of DNA replication. Temperature-induced recombinants have been shown to be meiotic, not oogonial, in origin; and marked sensitivity to temperature likewise appears to reside in the early oocyte rather than in the oogonia. The temperature responses of interstitial and proximal chromosomal regions reveal a chronological and a quantitative difference, with the later and most pronounced response in the region spanning the centromere. If temperature acts directly, exchange pairing is initiated at the interphase stage; whereas the excellent correlation between segregation behavior and mitotic metaphase length of noncrossover chromosomes suggests that distributive pairing occurs between greatly condensed chromosomes, late in the cycle.