Thermoregulatory mechanisms in rodents were found to respond to photoperiod manipulations. In desert adapted species, non-shivering thermogenesis NST-capacity (the ratio between the maximal VO2 response to noradrenaline injection and RMR, measured 1 degree C below the lower critical point) increased, as due to long scotophase acclimation. The aim of the present study is to compare the thermoregulatory responses of mesic rodents with those of desert ones, to photoperiod manipulations. Heat production, body temperature and NST were studied in the Levant vole, Microtus guentheri, the Migratory hamster, Cricetulus migratorius, and the Macedonian mouse, Mus macedonicus, acclimated to long (16L:8D) and short (8L:16D) photoperiod regimes at a constant ambient temperature of 25 +/- 1 degree C. The results of our study show that the three mesic species did not significantly change their NST capacity due to increase in the dark hours, as observed in desert species. However, in all three mesic species the increase in photophase resulted in a better resistance to high ambient temperatures by elevating the higher critical point and decreasing metabolic rates at the thermoneutral zone.