1. Haematological and respiratory parameters (including RBC count, CO2 content and percentage oxygen saturation) were studied in cold-acclimated (12 ± 1 °C) and normal frogs (23 ± 2 °C).
2. Seasonal variation in RBC count was found, the count being higher in January than in September. Females showed higher counts than males. Cold-acclimation resulted in the elevation of counts in all seasons and in both sexes, and was accompanied by a rise in haemoglobin content.
3. Cold-acclimated frogs retained a higher oxygen saturation in the truncus arteriosus than in the cutaneous vein, the carbon dioxide content of the truncus being also reduced in acclimated frogs. The O2/CO2 ratios were thus higher in the truncus than in the cutaneous vein.
4. Cold-acclimated frogs released more 14CO2 through the lungs.
5. It is suggested that a change in the mode of respiration occurs when the frogs are acclimated to cold.