Murashige & Skoog medium was modified for enhancing artemisinin production in Artemisia annua hairy root cultures by altering the ratio of NO3-/NH4+ and the total amount of initial nitrogen. Increasing ammonium to 60 mM decreased both growth and artemisinin accumulation in hairy root cultures. With NO3-/NH4+ at 5: 1 (w/w), the optimum concentration of total initial nitrogen for artemisinin production was 20 mM. After 24 days of cultivation with 16.7 mM nitrate and 3.3 mM ammonium, the maximum artemisinin production of hairy roots was about 14 mg l(-1), a 57% increase over that in the standard MS medium.