Measurements of nitrous oxide in the earth's atmosphere over the past 40 years indicate a concentration of between 250 and 400 ppb vol/vol N2O. There is some disagreement among researchers about the degree to which nitrous oxide varies at the surface, both on land and over the ocean. Measured variations in the surface concentration of N2O range from 10% to less than 0.5%, implying atmospheric lifetimes from 1.4 years to greater than 28 years, respectively. Recent extensive measurements by the authors and co-workers indicate extremely little variation of nitrous oxide in the troposphere, with a concentration of approximately 330 ppb vol/vol. Continuous measurements made over a 3-month period showed a variation of less than 1%. Samples collected from 80 ¿N to 90 ¿S latitude showed no significant variation. Measurements in the troposphere and lower stratosphere showed a constant level of N2O from the ground to the tropopause and then a gradual decrease in the lower stratosphere. The only large-scale variation found for nitrous oxide in the troposphere appears to be associated with the northeast trade winds system and may originate in North Africa. There also appears to be a bias inherent in the different methods of measuring N2O, and a program of intercalibration among the laboratories making measurements is necessary to resolve the question of the absolute concentration of nitrous oxide in the atmosphere.