In order in investigate the effects of osmotic stress on metabolism of NO3− in Lolium perenne L. cv. Réveille, plants were exposed to NaCl or polyethylene glycol 6000 in the culture medium. Four-week-old plants were kept in the dark for 36 h in order to decrease nitrate reductase activity (NRA). Afterwards, they were placed in a greenhouse for 27 h under continuous illumination with a nutrient solution containing 1 mM 14NH415NO3. To induce osmotic stress, the solution was supplemented with either NaCl or PEG 6000 to give an osmotic potential of —0·335 MPa in the medium. Osmotic adjustment in PEG treated plants was strongly limited, because PEG could not be absorbed. Over 70% of the osmoregulation in NaCl treated plants resulted from chloride uptake. Compared to control plants, PEG 6000 treatment decreased absorption (by 40%) and reduction (by 60 %) of NO3− in the leaves, leading to NO3− storage in the vacuolar pool. In contrast, addition of NaCl in the medium did not significantly affect either uptake or reduction of NO3−. The proportion of NO3− translocated to the leaves was unaffected by osmotic stress and represented about 76% of consumed NO3−. Nitrate reductase activity, measured in vivo with exogenously supplied NO3−, was not significantly different between treatments. Compared to reduction of p15NO3− in control plants, it appeared that reduction of NO3− in leaves of PEG-treated plants was not limited by enzyme activity or by the amount of reducing power. Availability of NO3− at the site of reduction therefore seems to be limiting when PEG 6000 is added to the medium.