Nitrous oxide (N2O) is an important atmospheric trace gas whose concentration is determined by numerous sources and sinks. Over the past 200 years, N2O has increased by 17¿3% due primarily to increased anthropogenic emissions. Over the last glacial/interglacial transition, data from ice cores show that N2O increased by 35%. This increase may be related to increased terrestrial N2O emissions resulting from a global climate amelioration. Here I describe a new dry extraction technique for measuring the elemental and isotopic composition of N2O in ice core samples. Results from a shallow GISP II ice core spanning the last 200 years provide an estimate for the preanthropogenic N2O concentration of 266¿4 ppb. In addition, 11 ice samples from the last interglacial period show fairly uniform N2O values (269¿9 ppb). During the penultimate glacial period (131--160 ka), N2O concentrations of air trapped in the glacier fluctuated between 190 ppb and 320 ppb, while CO2 and CH4 concentrations were relatively low and stable. Two local N2O maxima were observed during the penultimate glacial period that actually exceeded normal interglacial N2O values (~265 ppb). The maxima were located in portions of the core which exhibit both elevated dust concentrations and elevated bacterial counts. The Δ15N and Δ18O of N2O in this portion of the Vostok core were 15? higher and 12? lower than during the warm Eemian period, respectively. The isotopic data and coincidence between the elevated N2O concentrations and the high bacterial counts may be related to insitu N2O production by bacteria. The lower Δ18O values are consistent with the isotopic composition of N2O produced by in-situ nitrification. ¿ 2001 American Geophysical Union