The role of nitrate, ammonium, and culture medium pH on shoot organogenesis in Nicotiana tabacum zz100 leaf discs was examined. The nitrogen composition of a basal liquid shoot induction medium (SIM) containing 39.4 mM and 20.6 mM was altered whilst maintaining the overall ionic balance with Na(+) and Cl(-) ions. Omission of total nitrogen and nitrate, but not ammonium, from SIM prevented the initiation and formation of shoots. When nitrate was used as the sole source of nitrogen, a high frequency of explants initiated and produced leafy shoots. However, the numbers of shoots produced were significantly fewer than the control SIM. Buffering nitrate-only media with the organic acid 2[N-morpholino]ethanesulphonic acid (MES) could not compensate for the omission of ammonium. Ammonium used as the sole source of nitrogen appeared to have a negative effect on explant growth and morphogenesis, with a significant lowering of media pH. Buffering ammonium-only media with MES stabilized pH and allowed a low frequency of explants to initiate shoot meristems. However, no further differentiation into leafy shoots was observed. The amount of available nitrogen appears to be less important than the ratio between nitrate and ammonium. Shoot formation was achieved with a wide range of ratios, but media containing 40 mM nitrate and 20 mM ammonium (70:30) produced the greatest number of shoots per explant. Results from this study indicate a synergistic effect between ammonium and nitrate on shoot organogenesis independent of culture medium pH.