Genetic and physical analyses have demonstrated an intimate interaction or pairing of homologous chromosomes in the nuclei of many Drosophila cell types. Experiments were performed to determine whether P elements transposing from a given chromosome to its homolog would preferentially insert in the region corresponding to the donor site, perhaps due to such a proximity. A P [lacZ;ry+] element at the cactus locus (35F) on the second chromosome was mobilized and 96 insertions on the homolog were recovered. The distribution of these new insertions was determined by recombination mapping and molecular analysis, and compared with a control set of 93 second-chromosome insertions originating from the X chromosome. A nearly threefold preference was observed for re-insertion in a region of two to three number divisions around cactus on the homolog. However, none of these "local" insertions was actually within approximately 50 kb of the site at cactus corresponding to the starting site. This is in marked contrast to the previously described phenomenon of intrachromosomal local transposition, where the majority of local transpositions are within 10 kb. The data suggest that the mechanisms for intrachromosomal and interchromosomal local transposition are distinct, and are consistent with a model for interchromosomal local transposition involving proximity of homologous chromosomal regions in the nuclei of the germline cells.