The concentration of artemisinin in leaves of A. annua was at its highest in plants receiving between 6 and 56 mg L21 of nitrogen (N1–N3), where it ranged from 0.86 to 0.93% (w/w; Fig. 5). As nitrogen concentration increased from 56 to 106 mg L21 the concentration of artemisinin declined to 0.69% (w/w). A further increase in nitrogen application reduced artemisinin concentration to between 0.53 and 0.56% (w/w). However, the total yield of artemisinin (artemisinin concentration _ leaf biomass) per plant increased, from 0.053 to 0.31 g per plant, as the concentration of nitrogen supplied to the plants increased from 6 to 106 mg L21. Increasing nitrogen concentrations from 106 to 306 mg L21 did not increase artemisinin yields further.