Quantifying and simulating the relationships between crop growth, total‐nitrogen (total‐N) and nitrate‐N (NO3–‐N) concentration can improve crop nutritional husbandry. In this study, the relationship between shoot relative growth rate (RGR) and shoot total‐N, organic‐N and NO3–‐N concentration of hydroponically‐grown lettuce (Lactuca sativa var. capitata L. cv. Kennedy) was described and simulated. Plants were grown hydroponically for up to 74 d. Nitrogen was supplied throughout (control; T1), or removed at 35 d (T2) and 54 d (T3), respectively, after sowing. The organic‐N and NO3–‐N concentration declined in the shoots of control plants with growth, until commercial maturity approached when organic‐N and NO3–‐N concentration increased. There were sub‐linear relationships between both total‐N and organic‐N concen tration, and shoot RGR, in the N‐limited treatments, i.e. shoot RGR approached an asymptote at high shoot N concentration. The proportional effects of total‐N and organic‐N concentration on shoot RGR were independent of plant age. A dynamic simulation model (‘Nicolet’), derived previously under different conditions, was used to simulate the growth, dry matter content, organic‐N, and NO3–‐N concentration of lettuce grown under the extreme N‐stress conditions experienced by the plants. In view of the largely successful fitting of the model to experimental data, the model was used to interpret the results. Suggestions for model improvement are made.