1. From 1981 to 1986, 6120 meadow voles (Microtus pennsylvanicus) were sampled for hematological indices in southeastern Manitoba, Canada. This survey revealed the sporadic occurrence of anemia in early winter at mean temperatures below about -5 degrees C. 2. Anemia was associated with leukocytosis and circulating normoblasts, suggesting a sudden, large blood loss. Individuals became anemic quickly, with no obvious predisposing factors. 3. Attempts were made to induce anemia by exposing voles in traps to various temperatures. Temperatures characteristic of most trapping sessions failed to induce anemia in both wild and laboratory-born voles. 4. Short-term exposure to more extreme temperatures (-20 to -30 degrees C) induced anemia. Voles lost blood through erosions of the epithelium of the glandular stomach, and developed other pathological lesions characteristic of hypothermia. 5. Although there was a strong association between cold weather and anemia, we could find no clear relationship between winter survival and winter weather. However, in 1984, extraordinarily cold temperatures were associated with anemia and a subsequent population decline. These events suggest a threshold mean daily temperature of about -15 degrees C, below which vole survival is grossly affected. 6. Deteriorating protein levels and energy reserves of small mammals in winter may make them particularly susceptible to cold stress. Hence, sporadic bouts of sustained cold may be responsible for some of the enigmatic winter declines seen in northern small mammals.