Data from weekly global measurements of nitrous oxide from 1981 to the end of 1996 are presented. The results show that there is more N 2 O in the northern hemisphere by about 0.7+/-0.04 ppbv, and the Arctic to Antarctic difference is about 1.2+/-0.1 ppbv. Concentrations at locations influenced by continental air are higher than at marine sites, showing the existence of large land-based emissions. For the period studied, N 2 O increased at an average rate of about 0.6 ppbv/year (~0.2%/year) although there were periods when the rates were substantially different. Using ice core data, a record of N 2 O can be put together that goes back about 1000 years. It shows pre-industrial levels of about 287+/-1 ppbv and that concentrations have now risen by about 27 ppbv or 9.4% over the last century. The ice core data show that N 2 O started increasing only during the 20th century. The data presented here represent a comprehensive view of the present global distribution of N 2 O and its historical and recent trends.