Gamma-aminobutyric acid (GABA), a four-carbon non-protein amino acid, is a significant component of the free amino acid pool in most prokaryotic and eukaryotic organisms. In plants, stress initiates a signal-transduction pathway, in which increased cytosolic Ca2+ activates Ca2+/calmodulin-dependent glutamate decarboxylase activity and GABA synthesis. Elevated H+ and substrate levels can also stimulate glutamate decarboxylase activity. GABA accumulation probably is mediated primarily by glutamate decarboxylase. However, more information is needed concerning the control of the catabolic mitochondrial enzymes (GABA transaminase and succinic semialdehyde dehydrogenase) and the intracellular and intercellular transport of GABA. Experimental evidence supports the involvement of GABA synthesis in pH regulation, nitrogen storage, plant development and defence, as well as a compatible osmolyte and an alternative pathway for glutamate utilization. There is a need to identify the genes of enzymes involved in GABA metabolism, and to generate mutants with which to elucidate the physiological function(s) of GABA in plants.