We used capnometry during high-frequency oscillatory ventilation (HFOV), and compared CO2 measurements at the distal and proximal ends of an endotracheal tube with arterial CO2 values. Ten white rabbits (mean weight, 2.00 +/- 0.2 [SD] kg) underwent tracheostomy under anesthesia with pentobarbital. The trachea was intubated with an endotracheal tube with a second lumen for sampling respiratory gas at the distal tip. Capnometry was performed through the lumen (CO2d) and the proximal end of the endotracheal tube (CO2p). The internal carotid artery was cannulated to sample blood for measuring arterial blood gases. The differences between CO2d, CO2p, and PaCO2 were measured. Only the relation between CO2d and PaCO2 was good (r = 0.915). We concluded that capnometry can be used during HFOV to estimate PaCO2 provided that respiratory gas is sampled from the distal tip of the endotracheal tube.