When the submaxillary gland of the dog under morphine-urethane anesthesia was made to secrete at a constant rate by continuous intravenous administration of pilocarpine (1:100,000) increasing the CO2 in the inspired air increased the rate of secretion and decreasing the O in the inspired air decreased it. The increase in the rate was obtained with 1% CO2 in the inspired air. It was greater with higher percentages of CO2 (to 10%). It was greater the more prolonged the period of administration. Following the administration of CO2 when the animal was breathing room air again the secretion rate usually fell below that before the CO2 was given and recovered very gradually if at all. This after-effect was more marked with the higher % of CO2 or with prolonged administration. Decrease in secretion rate was obtained with 18% O in the inspired air. It was greater with lower % of O (to 7%). Following the low O period when the animal was breathing room air again the rate usually recovered promptly and might exceed that before the low O was administered. If the administration of 7% O was prolonged the secretion rate began to increase. In some cases this increase brought the secretion rate very much above the initial. The effects of high CO2 and low O were obtained with both chorda tympani and vago-sympathetic cut. They appeared to be independent of the effect upon the blood pressure of the artificial gas mixture administered and the level of the blood pressure at the time of administration, except that during low O inhalation the secondary increase in secretion rate and a fall in the blood pressure began at about the same time. The volume flow of blood from the sub-maxillary gland was increased both by increasing the CO2 and by decreasing the O in the inspired air.