Seasonal patterns of ozone (O₃) uptake in mature Norway spruce and cembran pine trees were estimated at six rural sites between 580 m and 1950 m a.s.l. Seasonal variations in O₃ uptake reflect variation in both ambient O₃ concentration and stomatal conductance. At all the study sites the annual course in the O₃ regime exhibited seasonal cycles with maxima between April and July and minima in winter. Seasonal variations in stomatal conductance and O₃ uptake were mainly attributed to the course of the prevailing temperature. Average O₃ uptake rates decreased with increasing tree age. However, in trees similar in age O₃ uptake increased with increasing altitude.