The mountain hare (Lepus timidus) is a year-round active herbivore adapted to survive the boreal winter. Captive mountain hares (N=4) were implanted with intraabdominal thermosensitive loggers to record their core body temperature (T(b)) for a year and during food deprivation (8-48h) in summer and winter. The average T(b) was 38.7+/-0.01 degrees C in summer and 38.3+/-0.01 degrees C in winter. The yearly T(b) correlated positively with the ambient temperature. The 24-h T(b) was the highest from late scotophase to early photophase in summer and winter and the lowest during middle-late photophase in summer or during early-middle scotophase in winter. The range of the 24-h oscillations in T(b) increased in three animals in winter. Food deprivation did not induce hypothermia in summer or winter. These preliminary data suggest that the mountain hare can spare a modest amount of energy with the wintertime reduction in T(b).