In a nutrient solution experiment with two Al (0 and 0.1 mM) combined with two Mg treatments (0.1 and 0.5 or 0.75 mM resp.) Al induced Mg deficiency in maize. Growth of maize responded to Mg only in the presence of Al, but did not attain the same level as without Al. Without Al in the nutrient solution 0.1 mM Mg was adequate. Contrary to growth the transpiration rate per unit leaf area was increased at the higher Mg concentration irrespective of the absence or presence of Al but was at an overall lower level in the latter case. It is assumed that Al acted through reducing Mg uptake and impeding root function with respect to water uptake.