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Threshold temperatures for shivering in acute and chronic hypercapnia were determined in guinea pigs by measuring the time course of cervical cord temperature, skin temperature, oxygen consumption (Vo2), and electrical muscle activity during cold exposure (15 degrees C). Prior to acute exposure to CO2, the shivering threshold was determined in each animal during control conditions breathing air. With increasing CO2 concentrations (5,7.5, and 15% CO2) the shivering thresholds fell to lower temperatures, decreasing by approximately 40 degrees C at 15% CO2. The shift of the shivering threshold to lower values found during acute exposure to 15% CO2 was reversed after chronic exposure to 15% CO2 for 3 days, which marks the time of metabolic adaptation to CO2.