Aluminium (Al) toxicity is considered an important factor in forest deterioration caused by soil acidification. A ratio of base cations (BC) to Al in the soil solution lower than 1 is widely used as an indicator for potentially adverse effects on tree health. In our view, the validity of the assumptions underlying the use of the BC:AI ratio as an indicator for Al toxicity in trees has never been evaluated properly. Here, we evaluate the importance of the base cations Ca and Mg in counteracting Al toxicity. Pinus sylvestris and Picea abies seedlings were grown on nutrient solution with a range of Al (0-0.25-0.5-1-2 mM) and base cation (0.25-0.5-2 mM) concentrations, giving BC:Al ratios of 1 at different levels of Al. Increasing concentrations of Al in solution caused growth reductions, which could not be counteracted by increasing concentrations of BC in solution with P. sylvestris and only partly counteracted with P. abies. Increased concentrations of Al in solution decreased the concentrations in shoot and root of both Ca and Mg, while increased concentrations of BC in solution increased tissue concentrations of BC. Growth reductions were, however, not a result of BC deficiencies, as growth reduction already occurred in tree seedlings that maintained adequate concentrations of Ca and Mg. All growth and uptake variables measured showed a higher or equal correlation with the absolute concentrations of Al or Al + BC in solution than with the BC:Al ratio. We conclude that Al toxicity is determined solely by the concentration of Al in solution. Shoot growth decreased significantly as dissolved Al increased at a constant BC:Al ratio of 1. In P. abies, but not in P. sylvestris, dissolved BC can positively affect uptake of BC and growth, which might partly alleviate the toxic effects of Al. Our results show that the mechanistic explanation for the effect of the BC:Al ratio is insufficient to describe Al toxicity. Care should be taken when using models based on the BC:AI ratio to predict the effect of Al on tree growth. (C) 2004 Elsevier B.V. All rights reserved.