A method is described that permits the recovery of a well-synchronized population of oocytes. Utilizing this pupal system, the heat-responsive period for increasing crossing-over in the Drosophila genome has been defined for the X chromosome and a portion of chromosome 2. The response is initiated close to the time of oocyte formation (premeiotic interphase) and is terminated after approximately 36 hr. During the 36-hr period different regions show characteristic responses, which vary in degree, in duration, and in initiation and termination points, so as to generate the beginning of a thermal recombination map for the Drosophila genome. Centromere regions exhibit the greatest increases in crossing-over for their respective chromosomes but are distinctly asynchronous in time; interstitial regions respond the least. Correlated autoradiographic studies have localized DNA replication in the oocyte to a approximately 24-hr period, which also begins close to oocyte formation (premeiotic interphase); late labeling in restricted regions, undetectable with the present method, could extend the period, as could prolonged synthesis in the oocyte. The results demonstrate that DNA replication and the heat-sensitive period for enhancement of crossing-over are coincident processes over most and possibly all of their length.