Respiration measurements at −7°, 0° and 7 °C. with four winter wheat varieties and two spring wheat varieties grown in the field, were carried on during the fall and early winter of four years. The rate of respiration in winter wheat decreased as the plants hardened. At the outset all varieties respired about alike. After hardening began, the rates at −7 °C. were in the inverse order of hardiness, with the spring wheats faster than the least hardy winter wheat. As hardening progressed, the differentiation of the winter varieties at −7 °C. became sharper, and could be observed also at 0 °C., but at 7 °C. very little if any relationship of this sort developed. The spring varieties, because of frost damage, could not be compared with winter varieties in the fully hardened condition, but their behavior early in the season confirms the general conclusion that at the freezing point or lower temperatures respiratory activity is inversely related to winter hardiness. This accounts for the observed fact that hardy varieties maintain their sugar reserves better than non-hardy varieties during the winter.