To determine the relative sensitivities of tomato, cucumber, and bean to exogenously applied concentrations of ferulic acid (FA) and to determine whether FA-induced stress responses increase endogenous levels of abscisic acid (ABA), wild-type andFlacca (ABA-deficient mutant) tomato (Lycopersicon esculentum Mill. cv. Ailsa Craig), cucumber, (Cucumis sativus L. cv. Early Green Cluster), and bean (Phaseolus vulgaris L. cv. Oregon 91) were treated with FA (0.0, 0.2, 0.4, 0.8 mM) in nutrient solution every other day for a total of two or three treatments. FA inhibited leaf growth and water utilization of wild-type tomato,Flacca tomato, and cucumber, but not of bean. Acclimation to FA was observed following the first FA treatment and increased endogenous ABA levels were found in wild-type tomato,Flacca tomato, and cucumber following multiple FA treatments. Induction of ABA biosynthesis occurred in wild-type tomato within 8 hr of FA treatment and maximum ABA levels were observed 24 hr after treatment. At that time, ABA levels of tomato treated with 0.4 and 0.8 mM FA were 13.7 times and 2.6 times higher than control levels, respectively. A second FA (0.4 or 0.8 mM) treatment, 48 hr after the first, did not appear to affect ABA levels. Ninety-six hours after the first treatment, ABA levels of tomato treated with 0.4 mM FA approached control levels; ABA levels of plants treated with 0.8 mM FA were 1.9 times higher than control levels. Control ABA levels increased gradually with time. The data showed that plant sensitivity and ability of subsequent acclimation to phenolic acids, such as FA, were taxa dependent.