The concentration of nitrous oxide has been measured in air samples collected monitoring stations and aboard oceanographic vessels in the major world oceans. These measurements demonstrate that the tropospheric nitrous oxide concentration is increasing at ~0.2% per year, thus confirming earlier observations of the increase based on stored samples. The measurements also show that the concentration of nitrous oxide in the northern hemisphere is higher than in the southern hemisphere, the average difference during the sampling interval having been about 0.8 parts per billion (ppb), compared to a January 1, 1978 northern hemisphere dry air mole fraction of 300.2 ppb. The data are well represented by a simple box model which relates the tropospheric rate of increase to an exponentially increasing source function. The observed increase may be explained by combustion of fossil fuels and agricultural activity, with a total source strength of ~11¿1010 mol/yr as of January 1, 1978. A substantial fraction of this production is explained by combustion, and agricultural production is therefore considerably less than has been previously estimated. The concentration of nitrous oxide in the preindustrial unperturbed troposphere is estimated to have been between 281 and 291 ppb, depending upon the rate of increase of the mean anthropogenic source function, and the preindustrial latitudinal distribution is estimated to have been nearly uniform. According to the model projections, the concentration of tropospheric nitrous oxide in the year 2000 will be 5 to 7% above present values. The observed rate of tropospheric increase directly affects the production of stratospheric nitric oxide, and plays a significant role in the earth's radiation balance, conservatively estimated as 10--15% of the effect due to increasing carbon dioxide.