At 17^oC recombination in rec³/+, rec³/rec³ and rec³/Df(3R)sbd105 is elevated. At 25^oC, the homozygote and the heterozygote display normal values, but recombination in the hemizygote is significantly reduced. At 31^oC, rec³/+ remains normal but rec³/rec³ recombination is drastically reduced. The hemizygote is sterile above 28^oC. In rec²/rec³ females which show the most extreme temperature responses, application of the restrictive temperature at sequential days during development at 25^oC shows control value activity to be reduced only with treatment on days six and seven and more precisely during a 36-hour period beginning at the time of pro-oocyte formation between 126 and 132 hours and terminating at 162 hours. This 36-hour period coincides with the boundaries of the S period as well as those of the temperature-sensitive period for enhancing recombination in the normal genotype. Electron microscopy of serially sectioned oocyte nuclei from rec²/rec³ females maintained at the restrictive or at the permissive temperature reveals synaptonemal complexes indistinguishable in length and fine structure, implicating recombination and not synapsis as the target of the rec gene product.