The dynamics of ammonium (NH4+) and nitrate (NO3-) concentrations in the soil solution is an important determinant of the species composition of natural vegetation. A mathematical model of uptake, assimilation and translocation of NH4+ and NO3- is presented to assess the performance of species with respect to NO3-/NH4+ feeding characterised by physiologically defined parameters. Nitrate efflux is explicitly considered. The capacities of NO3-, [ U], and NH4+ influx, [U], and NO3- reduction, [A], appear sufficient to characterise whole-plant N metabolism including NO3- translocation. The parameter space made up by these parameters is represented by 276 parameter combinations ('species'). Simulated total net N uptake rate and C costs for uptake and assimilation per mole total net N taken up are used to decide on how a species profits or suffers from NO3-+NH4+ mixtures relative to pure N forms with similar total N concentration for external concentrations up to 1.6 mM. Five response categories were identified and contrasted with categories defined by Bogner (1968) on the basis of experimental results on forest plants. The largest category comprises species that respond positively to NO3- and positively or indifferently to NH4+. These species have intermediate to high [U] and [A] and variable [U] and correspond to woodland edge species and forest plants on rich soil including typical 'nitrophilic' species. This category fades into a group of species that respond positively to NO3- and negatively to NH4+. These species have high [U] and low [U] and [A]; several species from oak-hornbeam woodland (Carpinion) belong to this group. Many parameter combinations were found that responded positively to NH4+ and indifferently to NO3-: low to medium [U], medium to high [ U] and variable [A]. This category includes all heathland species. No species were found which responded negatively to NO3-. The physiological background of differences between the categories is explained with respect to the equilibrium NO3- concentration in roots, influx, efflux, translocation and assimilation of NO3- and uptake and assimilation of NH4+. The relationship between NO3- accumulation capacity and morphology is discussed. Some slow-growing species with high [U] and low [A] use NO3- mainly as an osmotic solute. Respiratory costs in roots of inherently slow-growing species are discussed with respect to patterns in NH4+ and NO3- availabilities of their habitat.