After growth for 17 to 36 days on nutrient solutions with NH4NO3 as nitrogen source (pH 4.2) dry matter of sorghum genotype SC0283 was much less affected by A1 (1.5 and 3.0 ppm) than that of genotype NB9040.In the absence of A1 both cultivars released protons into the nutrient solution as a result of an excess of cationic nutrients taken up. When AI was present, this proton efflux per unit dry weight increased drastically, especially with the sensitive genotype NB9040. Chemical analysis of plant material and continuous analyses of NO3 and NH2-in the nutrient solution indicated, that the Al-induced shift in H +-balance of both genotypes could almost completely be attributed to a decreased NO3/NH, + uptake ratio.In vivo nitrate reductase activity (NRA) was reduced in the shoot of NB9040 and to a lesser degree in SC0283. Al-induced decrease in NRA was accompanied by similar percentual decreases in NO~-tissue concentrations. Therefore this decrease is interpreted as being indirect, i.e., the consequence of the reduced NO~-uptake of the plants.A direct repression of NRA by AI seems also unlikely because nitrate reductase activity of the roots (where cellular Al-concentrations should be higher than in shoots) was not affected in Al-treated plants of either genotype.