Using museum specimens, we studied temporal changes in skull size in two species of Japanese rodents, the large Japanese field mouse (Apodemus speciosus) and Pratt's vole (Eothenomys smithii = E. kageus) during the 20th century. We used the greatest length of the skull (GTL), zygomatic breadth (ZB), narrowest width of the skull across the interorbital region (IC) and the length of the upper cheek teeth row (M) as indicators to such changes. We found that GTL and ZB (but not IC and M) increased during the study period in mice, and that IC and M (but not GTL and ZB) increased marginally in voles. We attribute these changes to elevated ambient minimal temperatures, which increased food availability and energy savings for the mice, and required diet change in the voles. © 2004 The Linnean Society of London, Biological Journal of the Linnean Society, 2004, 82, 263–267.