The relationships among testes mass, sperm size (as indicated by sperm tail length), and body mass of Old World rats and mice were investigated. A near isometric relationship between testes mass and body mass was found but not between body mass and sperm size. In a few lineages, testes mass deviated considerably from the regression line with relatively very small testes occurring in Bunomys fratrorum, Bandicota indica, and B. savilei of Asia, Aethomys ineptus of Africa, some Pseudomys, and all Notomys of Australia. By contrast, relatively large testes mass occurred in Apodemus, Berylmys, and Maxomys bartelsii of Asia, Pogonomys of New Guinea, and some Australasian Rattus, Melomys, and Mastacomys. Considerable variability in sperm size was also evident; some species that had relatively small testes also had relatively small spermatozoa. The reason(s) for interspecific variation in relative testes mass and sperm size is unknown, but the data provide an opportunity to test the hypothesis that differences in relative testes mass, and perhaps sperm size, relate to interspecific differences in the amount of intermale sperm competition and in breeding systems.