The influence of an additional supply of ammonium to a nitrate containing hydroponic solution on the mineral nutrition of greenhouse rose plants (Rosa hybrida cv. ‘Lambada’) has been investigated. The effect on NPK uptake, mineral contents in roots and leaves as well as nitrate reductase (NR) and glutamine synthetase (GS) activities were examined. The addition of ammonium in a nutrient solution containing nitrate produced a total nitrogen uptake increase during shoot elongation, while in the absence of ammonium, nitrate uptake was lower during shoot elongation. Adding ammonium to the hydroponics solution caused an increase of potassium uptake while ammonium absorption was constant. Phosphate uptake was always higher in combined nitrate plus ammonium treatment, likewise in this treatment the most relevant effect on plant mineral content was the increase of P concentration in the roots. In both treatments with or without NH+4, free nitrate was notably higher in the roots than in leaves, indicating that nitrate reduction in rose plants takes place mainly in the leaves. NR activity in the leaves was higher when ammonium was present, whereas the root GS activity was similar in both treatments. The influence of ammonium on phosphate uptake and the subsequent effects on transport of other ions and enzymatic activities are discussed.