Anesthetized dogs were hyperventilated with various CO2-free gas mixtures containing 6-100% O2 at a rate of 16 1/min. for 4 minutes at normal and reduced body temperatures (28C). Arterial blood samples were withdrawn at the start of the first breath following the apneic period and analyzed for PCO2, PO2, and pH. The PaCO2 was used as an index of the respiratory threshold for PCO2 and designated as PTCO2. At both normal and reduced body temperatures the PTCO2 decreased when the PaO2 was lowered as a result of hyperventilation with a low oxygen gas mixture. The PTCO2 was highest when the PaO2 was highest following hyperventilation with 100% O2. The relationship between PaO2 and PTCO2 was alinear at both temperatures. During normothermia the slope of a plot of the mean values of the PTCO2-PaO2 relationship was steepest below a PaO2 of 35 mm Hg and in hypothermia the relationship was steepest below approximately 20 mm Hg PaO2. The results indicated no real apneic threshold for PaCO2 during normothermia or hypothermia but rather one which directly is related to the PaO2. (Author).