Alaskan ground squirrels exhibited a profound leucopenia during hibernation and hypothermia. The total number of erythrocytes declined slightly also but the differences could not be established as statistically significant. The packed cell volume values were significantly different from normal animals only during hibernation. Hemoglobin measurements revealed a slight, though not significant, drop during hibernation. Measurements of complement titers in the serum withdrawn from active, hibernating and hypothermic animals suggested that significant differences in complement activity did not accompany changes in body temperature during either hibernation or hypothermia. Electrophoretic patterns of sera from hibernating ground squirrels revealed the presence of a new peak in the vicinity of the beta-globulin component. The appearance of this peak was shown to correspond with the onset of hibernation and its disappearance with arousal of the animal from hibernation.