The purpose of the experitnental study was to identify mechanical and/or neural factors contributing to the heart rate alterations associated with endotracheal auctioning (ETS). Effects of catheter insertion alone and catheter insertion with the application of negative pressure on the partial pressure of arterial oxygen (PaO2) and heart rate prior to and following vagal blockade with atropine were examined. Eleven newborn swine less than 24 h of age were randomly exposed to (a) suction catheter insertion alone and (b) suction catheter insertion and the application of suction. Preoperatively, the subjects were anesthetized with ketamine and xylazine (.05 mL/kg). The ECG in lead II was recorded on a Gould 2600S recorder. Arterial Wood (aB) O2levels were withdrawn 1 min before and at 20 s, 1 min, 2 min, and 3 min following ETS and analyzed on a calibrated Corning 158 semi-automated aB gas machine. There was a significant decline in PaO2from baseline at 20 s post ETS in both the no suction and suction protocols, p < .05. The application of suction caused a greater decline in mean PaO2(41 torr) compared to no suction (13 torr) at 20 s post ETS which was similar in pattern following vagal blockade (suction: 37 torr; no suction: 2 torr). Following the first pass of the catheter, mean heart rate (HR) declined 14 beats per min with suction and 9 beats per min with no suction. The difference was obliterated following the second pass of the catheter. There was no difference in the HR following vagal blockade.