Abstract: DL-b-aminobutyric acid (BABA) is a non-protein amino acid that confers widerangingprotection in a number of plant families against a variety of plant pathogens and insectherbivores. However, BABA can sometimes produce phytotoxic effects or lead to reductions inplant growth or yield. The performance of insect herbivores maintained on BABA-treated hostplants could be affected by stress-induced changes in the plant as well as by enhanced plantdefences, so it is important to clarify the effects of BABA on plant functioning. Soaking seeds(Vicia faba, Medicago truncatula, Brassica oleracea, B. rapa and Hordeum vulgare) in BABAsolution for 24h had no effect on their subsequent germination. However, application of BABAas a root drench caused reductions in shoot length and shoot fresh weight of Pisum sativum,Medicago truncatula, Beta vulgaris, Hordeum vulgare, Solanum lycopersicum and Brassicaoleracea. BABA consistently induced an increase in shoot percent dry weight, suggesting aneffect on plant water balance. BABA caused numerous changes in the chemical composition ofshoot material of Vicia faba and Medicago truncatula, including an increase in the concentrationsof H and N. There was also a decrease in K in both plants, which might be another factorassociated with plant water balance. The results suggest that, although BABA is known to act viathe priming or activation of various defence pathways, there are also a number of physical andchemical changes induced in the host plant that could potentially impact on the performance ofinsect herbivores.