The variations in glutamate decarboxylase activity and in glutamic acid and ?-ABA concentration have been measured in barley embryos during the uptake of water and in the roots and shoots for up to 6 days of growth. Glutamate decarboxylase activity was relatively steady in the embryos during soaking but rose rapidly once growth began. This development paralleled an increase in the concentration of glutamic acid in both roots and shoots at a time when the concentration of ?-ABA was falling. During soaking in aerated water, the ?-ABA content of the embryos rose for 36 h, at which point it accounted for 35 per cent of the soluble amino acids. ?-ABA was found to be a major free amino acid in roots but not in shoots. Experiments in vivo involving 14C-labelled glutamic acid and ?-ABA indicated that carbon from ?-ABA passed very rapidly into the citric-acid cycle intermediates and also that, throughout the period studied, ?-ABA was formed from glutamic acid despite the alterations in relative concentrations of these amino acids in the growing tissue. © 1971 Oxford University Press.