Because of discrepancy in reports concerning the rate of uptake of nitrous oxide by man, it seemed desirable to measure this factor when automatic control of concentrations and rapid computer measurement of breath to breath uptake became available. Six subjects inhaled 25 per cent nitrous oxide for two hours. Total uptake and alveolar uptake were determined. The total inhaled uptake, adjusted for a 70 kg subject inhaling 75 per cent nitrous oxide, corresponded well with an absorption of about 1000 ml the first minute. After a wash-in period of less than four minutes, the alveolar uptake, which at that time and subsequently equaled the total uptake, decreased approximately according to the square root of time in minutes. Considerable variation appeared between individuals, but each course approximated the same type of decrease. Absorption continued to occur during the two hour period of observation.