The behaviour of137Cs from the Chernobyl nuclear power plant accident in the Ukraine on 26 April 1986 was monitored during a 3-year period in the canopy of a young Scots pine (PinussylvestrisL.) stand in southern Finland. Needle samples were collected from the same whorls on 13 sample trees marked at the beginning of the study. The137Cs activity of throughfall water was determined in summer 1986. The oldest needles at the time of the radioactive deposition (formed in 1983) had the highestl37Cs activity (20 000 Bq/kg) in June 1986, when the monitoring was started, 58 days after deposition. About 50% lower137Cs activity was observed in the needles formed in 1984 and 1985, and an even lower activity of 3050 Bq/kg in the needles formed in 1986 following the incident. During the summer of 1986, the137Cs activity of the whole canopy decreased from 106 000 to 42 500 kBq/ha, corresponding to a weathering half-life of 72 days. Washout by throughfall accounted for 79% of the decrease, while the rest was mainly due to the gradual shedding of the highly contaminated 1983 needles. The needles formed in the summers of 1987 and 1988 had rather high137Cs activities of 2700 and 4800 Bq/kg, respectively. The reduction in the137Cs activity of the canopy was considerably less in 1987 and 1988, with weathering half-lives of 435 and 242 days, respectively. The total137Cs activity of the canopy was still 13 500 kBq/ha on 20 October 1988, which was 896 days after deposition. An increase in the137Cs activity was observed in the autumns 1987 and 1988, and to a lesser degree in the autumn of 1986, a time of year when needle potassium concentrations normally tend to increase. It was concluded that the Chernobyl137Cs had become mixed with the chemically analogous K, and was recirculated within the trees together with K.