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The rate of root elongation in seedlings of Picea abies was drastically inhibited by exposure to AlCl₃ at 800 or 1200 μmol/dm³ in nutrient solutions. A 35-day exposure to Al at 700 μmol/dm³ reduced Mg and Ca in roots and needles of the seedlings. The Mg content of needles was reduced to levels considered to be critical for Mg deficiency. In investigations of ⁴⁵Ca uptake into roots, exposure to Al at 100-800 μmol/dm³ reduced ⁴⁵Ca uptake by 77-92%, respectively. By using x-ray microanalysis, the distribution of Al, Mg, Ca, and K was found to be similar in roots of Picea abies seedlings grown in solution culture and in roots collected from declining spruce stands at Solling, F.R.G. In solution culture Al displaced Mg and Ca in the root cortex. A mechanism of Al toxicity for root growth and ion uptake is proposed, and its relevance to forest decline is discussed.