In 2007/2008 testes size of edible dormouse (Glis glis) kept in enclosure (35 individuals) and free-living in the woodland (22 individuals) were determined during active season by the two methods, caliper and ultrasound. In both environments edible dormice showed, as expected, strong seasonal fluctuations in their testes size, with the maximum reached in June, the time of mating. Adults developed significantly bigger testes than yearlings over the active season. Field individuals had significantly bigger testes than enclosure-housed animals and showed a 15 times bigger chance of palpable testes rate then edible dormice in enclosure. A common ideal testes size for both age-classes, adults and yearlings, was found immediately before the beginning of hibernation in autumn. Comparison of body mass between both environments revealed that field individuals were significantly lighter than enclosure-housed animals. As this study was carried out to compare the reliability of caliper and ultrasound for testes development in edible dormice the examination with the ultrasonoscope LOGIQ e turned out to be more accurate.