ABSTRACT: Many vegetables, especially leafy vegetables, accumulate NO−3-N in their edible portions. High nitrate levels in vegetables constitute a health hazard, such as cancers and blue baby syndrome. The aim of this study was to determine if (1) ammonium nitrogen (NH+4-N) and glutamine-nitrogen (Gln-N) absorbed by plant roots is converted into nitrate-nitrogen of pak-choi (Brassica chinensis L.) tissues, and (2) if nitrate-nitrogen (NO−3-N) accumulation and concentration of pak-choi tissues linearly increase with increasing NO−3-N supply when grown in nutrient solution. In experiment 1, 4 different nitrogen treatments (no nitrogen, NH+4-N, Gln-N, and NO−3-N) with equal total N concentrations in treatments with added N were applied under sterile nutrient medium culture conditions. In experiment 2, 5 concentrations of N (from 0 to 48 mM), supplied as NO−3-N in the nutrient solution, were tested. The results showed that Gln-N and NH+4-N added to the nutrient media were not converted into nitrate-nitrogen of plant tissues. Also, NO−3-N accumulation in the pak-choi tissues was the highest when plants were supplied 24 mM NO−3-N in the media. The NO−3-N concentration in plant tissues was quadratically correlated to the NO−3-N concentration supplied in the nutrient solution.