In situ measurement of atmospheric nitrous oxide (N2O) has been carried out at Hateruma monitoring station (lat 24°03'N, long 123°48'E) since March 1996 by the National Institute for Environmental Studies (NIES). A fully automated gas chromatograph equipped with an electron capture detector (ECD) measures the N2O concentrations at a frequency of 3 air samples per hour. Details of the experimental methods and procedures are presented in this paper. The N2O concentrations observed from March 1996 to February 1999 increased at an average rate of 0.64 ppb/yr. The observed data also suggest that there is a weak annual cycle of N2O concentration, increasing in autumn and winter and decreasing in spring and summer, with a peak-to-peak amplitude of at most 0.3 ppb. The N2O mixing ratios, smoothed with the 24-h running average, clearly showed short-term variability with synoptic timescales and had peak-to-peak amplitudes of about 1 ppb or less. These short-term variations correlated positively with the short-term variations of CO2 during the period from winter to early spring when the air masses arriving at Hateruma are dominantly transported from the Asian continent. The ΔN2O/ΔCO2 ratios could be used to constrain the relative strengths of these fluxes on a regional scale. (C) 2000 Elsevier Science Ltd.