Continuous ozone concentration measurements at rural sites in Lithuania have been performed since 1982. Long-term ozone data show an increasing trend 0.4 ppb y(-1) in annual mean concentrations during the period 1982-2008, although ozone peak values show an insignificant tendency to decrease. Several studies were performed to evaluate the ozone effect on the main plant species of Lithuanian forests. A strong positive correlation was found between values for the accumulated doses over a threshold of 40 ppb index and the defoliation of Fraxinus excelsior trees in Lithuanian forests during 1991-2008. A strong correlation was found between ozone exposure and the proportion of healthy F. excelsior, Betula sp., and Alnus incana, trees indicating that in Lithuania the deciduous species are more sensitive to ozone than conifers and that healthier trees may be better indicators of the changing environment. The results of the studies suggest that the observed levels of ozone are sufficiently high to have negative effects on the vegetation in Lithuania.