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In September 1990, samples of wood and bark were collected from Pinus sylvestris L. at three locations exposed to different levels of radioactive fallout from the 1986 accident at the Chemobyl nuclear power plant (NPP). Cross-sections of wood from the most exposed location showed a distinct change in histology in the annual ring of 1986, a consequence of the accident on 26 April. The width of annual rings decreased after the accident, and the relative width of latewood in annual rings increased transiently in 1986 and subsequently decreased in 1987. In 1987, an increase in the number of vertical resin ducts was observed, related to contamination at the location, and the number of radial rays decreased at the two locations of higher contamination. The radionuclide content in the bark was found to correlate with the degree of damage in the wood. There are several hypotheses about the contribution from various types of radioactive contamination, but the results indicate that both 'cloud gamma' and deposited radioactivity (beta and gamma) were of importance. The present work suggests that detailed studies of dose-effect relationships after exposure to different dose rates and radiation qualities may establish the usefulness of pine trees as in situ, time-recording differential dosimeters of ionizing radiation.