[en] This report was drawn up by the National Institute of Radiological Sciences. In order to measure tritium levels and consequently to assess the significance of the tritium in the environmental water around nuclear facilities regarding public health, the tritium surveillance program began in 1967 locally in Tsuruga and Mihama districts. Now, it has been expanded to 12 nuclear power stations and 5 nuclear facilities. In this report, the data from 1977 to 1980 are presented. Though the frequency of sampling was limited to once or twice a year in most cases, the time and point of measurement were fixed in each station in order to know the annual variation of tritium levels. The water samples were land water such as rivers, reservoirs and ponds and wells, sea water for secondary cooling system, and tap water around nuclear facilities, and land water, sea water and tap water in the places remote from nuclear facilities as the criteria. The samples with less than 100 pCi/l of tritium were electrolytically enriched, and counted with liquid scintillation counters. The results in respective facilities are reported. The tritium concentration in rivers, streams and reservoirs decreased exponentially from 400 pCi/l in 1971 to 100 pCi/l in 1980 with the half life of about 5 years in northeast Japan. Sea water was not affected by the effluent from nuclear facilities, and the tritium concentration in tap water was almost same as rivers. (Kako, I.)