Seasonal patterns of ozone (O-3) 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 O-3 uptake reflect variation in both ambient O-3 concentration and stomatal conductance. At all the study sites the annual course in the O-3 regime exhibited seasonal cycles with maxima between April and July and minima in winter. Seasonal variations in stomatal conductance and O-3 uptake were mainly attributed to the course of the prevailing temperature. Average O-3 uptake rates decreased with increasing tree age. However, in trees similar in age O-3 uptake increased with increasing altitude.