The rate of indole-3-butyric acid (IBA) synthesis in maize seedlings is dependent on the culture conditions of the plants. When the seedlings were grown on filter paper soaked with different amounts of water, the activity of IBA synthetase differed strongly. High amounts of water (150 and 200 ml per bowl) inhibited IBA synthesis completely in vitro, whereas 30 and 50 ml water per bowl increased the activity dramatically. Under conditions where IBA synthetase was inhibited (150 ml H₂O), an increase of enzyme activity was observed when abscisic acid (ABA) was exogenously added in concentrations between 5×10^-4 to 5×10^-7 M. Under 'drought' conditions (50 ml H₂O per bowl) the same ABA concentrations were inhibitory. Jasmonic acid and salicylic acid also enhanced IBA synthetase activity to some extent, whereas indole-3-acetic acid (IAA) and kinetin had no effect. Activity could also be enhanced by osmotic stress (NaCI and sorbitol), but not under temperature stress. In accompanying investigations the endogenous contents of IAA, IBA, and ABA under the different culture conditions have been determined as well as the energy charge of the seedlings. Similar observations have been made with Amaranthus, wheat and pea seedlings