A long-term study of concentrations of NH 3 and particulate NH + 4 has been performed at Hiyoshi, in Yokohama. From January 1987 to December 1991, daily measurements of NH 3 and particulate NH + 4 were made simultaneously five times a month. The results were discussed from the standpoint of seasonal variations and annual trends, meteorological effects, and special characteristics. The monthly mean NH 3 concentrations were in the range of 2.4-11.9 ppb and showed regular seasonal variations with the highest concentrations during summer and the lowest during winter. The seasonal trends seemed to be largely affected by air temperature. The NH 3 concentrations increased exponentially with increasing air temperature, and a high correlation was obtained (r=0.74). The NH 3 concentrations were not affected by wind direction and velocity. The monthly mean concentrations of particulate NH + 4 were within the range of 1.5-3.1 μg m - 3 , and the variation in the range was relatively small. The NH + 4 concentrations decreased steeply with increasing wind velocity. For the five-year period, there were no long-term trends in the concentrations of NH 3 and particulate NH + 4 .