Panting in animals can be expected to represent a naturally occurring physiological counterpart to today's techniques of mechanical high-frequency ventilation. To analyze the mechanisms underlying the gas exchange inefficiency during ventilation with high frequencies, steady-state pulmonary gas exchange was studied in seven conscious dogs (32 kg mean body weight) during panting elicited by mild thermal stress. The animals had a chronic tracheostomy and an exteriorized carotid artery loop and were exposed to 27.5 degrees C ambient temperature for 2 h (65% relative humidity). Open-circuit techniques were used and PO2 and PCO2 from the tracheostomy tube were continuously monitored by mass spectrometry using a special sample-hold phase-locked gas sampling technique. PO2 and PCO2 were determined in arterial blood collected from the carotid artery. During the exposure, the following variables of steady-state gas exchange were determined (means +/- SD): breathing frequency 313 +/- 19 min-1; tidal volume, 167 +/- 21 ml; total ventilation, 52 +/- 9 l.min-1; effective alveolar ventilation, 5.5 +/- 1.3 l.min-1; partial pressures (torr; a, arterial; E', end-tidal): PaO2, 106.2 +/- 5.9; PaCO2, 27.2 +/- 3.9; (PE'-Pa)O2, 26.0 +/- 5.3; (Pa-PE')CO2, 14.9 +/- 2.5. According to the conventional lung model, parallel-dead space ventilation (ventilation of unperfused lung regions) would account for about 55% of the alveolar ventilation and for 2/3 of the (PE'-Pa)O2 difference. However, the lack of an 'alveolar plateau' in the CO2 and O2 expirograms suggests that incomplete serial mixing in peripheral airways contributes to the enhanced gas exchange inefficiency during panting as reflected in the increased blood/gas differences for O2 and CO2.