Endogenous RNA polymerase activity has been characterized in nuclei isolated from embryo axes of Acer platanoides. Optimal activity was recorded at 4·0 mol m MgCl and 50 mol m (NH) SO and total activity could be inhibited by up to 30% by α-amanitin. Stratification of fruits leads to a stimulation of RNA polymerase activity. A minimum of 3 d cold treatment is required with at least 3-fold stimulation recorded after 10 d at 4°C. The increased enzyme activity is resistant to α-amanitin suggesting an effect on RNA polymerase I.