Two cultivars of Lupinus angustifolius L. were grown in a glasshouse in solutions containing NO3-, NH4+ or NH4NO3 with a total nitrogen concentration of 2.8 M m-3 in each treatment. One cultivar chosen (75A-258) was relatively tolerant to alkaline soils whereas the other (Yandee) was intolerant to alkalinity. Controlled experiments were used to assess the impact of cationic vs. anionic forms of nitrogen on the relative performance of these cultivars. Relative growth rates (dry weight basis) were not significantly different between the two cultivars when grown in the presence of NO3-, NH4+ or NH4NO3. However, when NO3- was supplied, there was a modest decline in relative growth rates in both cultivars over time. When plants grown on the three sources of nitrogen for 9 days were subsequently supplied with 15NH4NO3 or NH415NO3 for 30 h, NH4+ uptake was generally twice as fast as NO3- uptake, even for plants grown in the presence of NO3-. Low rates of NO3- uptake accounted for the decrease in growth rates over time when plants were grown in the presence of NO3-. It is concluded that the more rapid growth of 75A-258 than Yandee in alkaline conditions was not due to preferential uptake of NH4+ and acidification of the external medium. In support of this view, acidification of the root medium was not significantly different between cultivars when NH4+ was the sole nitrogen source.