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Venous gas emboli are prevented from reaching the systemic circulation by filtration in the pulmonary vasculature. This filtration can be overwhelmed by exceeding certain critical rates of venous air infusion. To characterize further these filtration phenomena, the effects of pentobarbitone, isoflurane and halothane anaesthesia on the incidence of spillover of venous bubbles into the arteries were studied in groups of nine dogs. Venous air was infused at rates of 0.25, 0.30, and 0.35 ml kg-1 min-1. Spillover of venous bubbles into the arteries was detected with a Doppler ultrasonic probe located over the suprarenal aorta. At the lowest venous air dose (0.25 ml kg-1 min-1), no bubbles were detected in the systemic circulation in the pentobarbitone- or halothane-anaesthetized dogs, while arterial bubbles were detected in two with isoflurane anaesthesia. At 0.30 ml kg-1 min-1 air, one, four and two dogs had arterial bubbles detected with pentobarbitone, halothane or isoflurane anaesthesia, respectively, while at 0.35 ml kg-1 min-1 spillover of bubbles occurred in four, five and three, respectively. The spillover of venous bubbles into the arteries was dose-related for the pentobarbitone- and halothane-anaesthetized dogs.