The effect on stomatal closure by ABA and its analogues, WL19224 and WL19377 was investigated. The rate of closure showed a sigmoid curve when various concentrations of ABA were applied. A concentration of 10-9 M ABA was the threshold for stomatal closure; maximal closure occurred at higher concentrations (from 10-6 M to 10-3 M). Use of the analogue WL19224 resulted in similar closure responses. However, ABA was more effective at lower concentrations. For example, at 10-3 M of either WL19224 and ABA, stomatal closed to 2.2 ${mu}{ extrm}{m}$ and about 3 ${mu}{ extrm}{m}$, respectively. In contrast, applications of the ABA analogue WL19377 had no effect on stomatal closure. In fact, at concentrations of WL19377 higher to 10-4 M, stomata were stimulated to open, to about 10% of their initial size. Likewise, applications of WL19377 along with ABA, inhibited ABA-induced stomatel closure. This inhibition was linearly related to the concentrations of the compounds applied. In conclusion, the structural requirements for biological activity of ABA and its analogues cannot be considered individually, but must be assessed for their roles as part of an entire functional group. Although compounds may have similar structures, their ability to control certain physiological activities may be quite different.