Abstract A simulation model was developed to estimate the stomatal conductance and ozone flux to Norway spruce saplings in open-top chambers. The model was parameterized against needle conductance measurements that were made on 4–6-year-old spruce saplings, grown in open-top chambers, in July–September during three different seasons. The spruce saplings were either maintained well watered or subject to a 7–8 week drought period in July–September each year. The simulated conductance showed a good agreement with the measured conductance for the well-watered as well as the drought stress-treated saplings. The simulations were significantly improved when different vapour pressure deficit (VPD) functions were applied for well-watered and drought-stressed spruce saplings. The cumulated ozone uptake which was calculated from the conductance simulations showed less variation between years, compared to the cumulative ozone exposure index AOT40 (accumulated exposure over a threshold of 40 ppb or nl l −1) for the corresponding time periods. Measurements in May 1995 demonstrated the occurrence of long-term ‘memory-effects’ from the drought stress treatments on the conductance. Memory-effects need to be considered when simulation models for stomatal conductance are to be applied to long-lived forest trees under a multiple stress situation.