Greenhouse-grown tobacco plants of the speciesNicotiana sylvestris (Solanaceae) subjected to leaf damage show a fourfold increase in the alkaloid content of their undamaged leaves. This increase in nicotine and nornicotine concentrations begins 19 hr after the end of the damage regime, reaches a maximum at nine days, and wanes to control levels 14 days after the start of leaf damage. The increase in leaf alkaloid content in damaged plants is largely due to a 10-fold increase in the alkaloid concentration of the xylem fluid entering leaves, which, in turn, suggests that increased synthesis of alkaloids is occurring in the roots. This research distinguishes between positive and negative cues affecting the change in xylem fluid alkaloid concentrations. A negative cue, such as auxin, when lost or diminished as a result of leaf damage could signal the alkaloidal response. Indeed, exogenous applications of auxin to damaged leaves inhibit the alkaloidal response. However, attempts to block endogenous auxin transport by steam girdling or applying an auxin transport inhibitor fail to mimic the effect of leaf damage on leaf alkaloid concentrations. The damage cue appears to be a positive cue that is related to the timing and the amount of leaf damage rather than to the amount of leaf mass lost. Moreover, when performed proximally to leaf damage, steam girdling truncates the alkaloidal response. This induced alkaloidal response appears to be triggered by a phloem-borne cue that allows the plant to distinguish between different types of leaf damage. The physiological and ecological consequences of the mechanism of this damage-induced alkaloidal response are further explored.