We have studied the problem of pulmonary capillary-alveolar CO2 exchange in the cat during acute hypercapnia. Three cats, anesthetized with xylazine and pentobarbital sodium and prepared with acute tracheostomy and femoral arterial catheter, and three awake cats, prepared with a small tracheal catheter and femoral arterial catheter, were subjected to acute hypercapnia (FICO2 = 0.00, 0.06, and 0.08). During steady states, end tidal PCO2 was determined with an infrared analyzer, and arterial PCO2 was measured with a Radiometer ABL-2 analyzer in simultaneously drawn samples. In vitro studies indicated that our blood sampling techniques resulted in a 6% reduction in PCO2. Blood PCO2 readings were corrected for (1) non-ideal performance of the analyzer as determined by tonometry, (2) 6% reduction due to sampling, and (3) differences between electrode and rectal temperature. Mean arterial-end tidal PCO2 differences were not significantly different from zero in any condition in either group (except for 0.08 CO2 in the awake group when the difference was 2.0 Torr). These findings in the cat agree with the classical view that PCO2 in pulmonary capillary blood approaches PCO2 in alveolar gas. Further, our findings provide evidence that CO2 loss from blood samples is an important technical factor which can cause systematic underestimation of blood PCO2 and, hence, contribute to the observation of negative PCO2 gradients.