This study was undertaken to examine the trade–off between the cost of thermoregulation and immune function in laboratory mice. Mice were maintained either at 23°C or cold exposed at 5°C for 10 days. Then, they were immunized with sheep red blood cells. Thus, the cold–exposed mice had either experienced or not experienced cold stress prior to immunization. Cold stress elicited a substantial increase in food intake, accompanied by a significant reduction in food digestibility. An increase in mass of metabolically active internal organs (small intestines, heart and kidney) was observed in cold–exposed mice. These findings reassured us that costs of increased thermoregulation caused by cold stress were substantial. The immune response of mice exposed to long–lasting cold stress was significantly lower, but immune response was not affected in short–exposed mice. Differences in immune response between experimental groups accompanied changes in mass of immunocompetent organs (thymus and spleen). Our findings indicate that studies of trade–offs should account for the fact that resource reallocation in response to an environmental challenge may not be immediate. In fact, resource reallocation may be postponed until the new environmental state becomes permanent or until an organism attains physiological adaptation to the current conditions.