This paper investigates the influence of seasonal adaptations to thermoregulatory heat loss for deer mice (Peromyscus) during summer and winter. A general, mechanistic model of heat transfer through fur was evaluated for the structural properties of the fur of deer mice. The model was validated against heat production determined from mice exposed to a range of radiative (wall) temperatures (tr) at air temperatures (ta) of 15, 27 and 34 degrees C. Calculated heat loss from the appendages was subtracted from the measured heat production to yield heat loss from the furred torso. This calculated torso heat loss agreed closely with the predicted fur heat loss for all conditions, as shown by a regression slope near 1 (0.99). Simulations using models of fur and appendage heat loss reveal that the winter increase in thermogenic (heat production) capacity has a greater effect than changes in fur properties in expanding the limits to thermoregulation. Both wind and a clear night sky increase heat loss and can limit thermoregulation to air temperatures above those found in deer mice habitats during winter (-25 degrees C). Thus, despite seasonal adaptations, these simulations indicate that thermoregulation is not possible under certain winter conditions, thereby restricting deer mice to within the protected environment of the leaf litter or snow tunnels.