The type of bedding material has been reported to affect the environment in both the animal cage and animal room. It has an impact on the health and well-being of the animals and may cause biased experimental results. Requirements for bedding materials, particularly those regarding animal comfort are poorly supported by experimental data. In this study, various types of bedding material were evaluated using preference tests with mice and rats. It was found that beddings consisting of relatively small particles (< or = 1.2 x 1.6 mm2) were generally avoided, whereas beddings consisting of large fibrous particles were preferred. The characteristics of the bedding materials were further investigated by scanning the size and shape of the particles, and by the assessment of ultrasound produced by the moving of the beddings. The results seem to indicate that size manipulability are among the main determinants of the appreciation of bedding particles by laboratory mice and rats, and larger particles are preferred.