A simple three equation model is proposed for the feedback regulation of nitrate uptake and N2 fixation, based on the concentration of the organic N substrate pool within the plant and two parameters denoting the N substrate concentrations at which half-maximal inhibition occurs. This model simulated three contrasting phenotypes of white clover (Trifolium repens L.) inbred lines with (1) normal rates of nitrate uptake and N2 fixation (NNU); (2) low rates of nitrate uptake (LNU); and (3) very low rates of N2 fixation (VLF). The LNU phenotype was simulated by a decrease in the value of the inhibition parameter for nitrate uptake and the VLF phenotype was simulated by a decrease in the value of the N2 fixation inhibition parameter. The model was tested against nitrate uptake data obtained from white clover plants growing in flowing nutrient culture. There was an accurate prediction of the increase in nitrate uptake caused by N2 fixation activity of the NNU and LNU inbred lines being interrupted by a switch in gas phase from air to Ar : O2. The model was also tested against data for nitrate uptake, N2 fixation and %N from fixation for the three inbred clover lines grown in flowing nutrient culture at 0, 5 or 20 mmol m–3 NO3–. Again there was accurate prediction of nitrate uptake, although simulated values for N2 fixation were more variable. The simple model has potential use as a sub-routine in larger models of legume growth under field conditions.