The effect of nitrate concentration on uptake and assimilation of nitrate by dark-grown, decapitated maize ( Zec mays L.) seedlings was examined. During the 12-h exposure period, twice as much nitrate was taken up from 20 mM as from 0.05 mM K 15NO 3 (98.7 atom% 15N). The higher concentration had little effect on nitrate reduction, but nitrate translocation was more than doubled. Thus, partitioning to translocation accounted for 80% of the increased uptake which occured from 20 mM nitrate. At both concentrations a relatively constant percentage, 22–27%, of the reduced- 15N was incorporated into protein and other insoluble nitrogenous compounds. Concurrently, translocation of soluble reduced- 15N increased from less than 10% of the total reduced- 15N at 4 h to more than 50% at 12 h. Neither the translocation of reduced- 15N nor its incorporation into protein was affected appreciably by the 400-fold difference in exogenous nitrate concentration. The results may be interpreted in one of two ways. At 0.05 mM, uptake occurred mainly via ‘mechanism 1’ of ion uptake, whereas at 20 mM, ‘mechanism 2’ also contributed: nitrate reduction was predominantly associated with uptake via the former mechanism and nitrate translocation with the latter. Alternatively, it is possible that at the low nitrate concentration uptake occurred primarily via root cells which contained nitrate reductase (NR), whereas at the higher concentration, uptake occurred in addition via cells which were dificient in NR and from which translocation of unreduced nitrate was thus more likely.