De-embryonated grains of wheat and barley showed various responses when incubated with gibberellic acid (GA3). Amylase activity in the three cultivars studied showed similar responses to GA3 concentration and period of incubation. Changes in o-diphenolase and monophenolase activities of polyphenol oxidase (o-Diphenol: oxygen oxidoreductase, E.C.I. 10.3.1) which occurred when de-embryonated grains were incubated in the absence of GA3, were enhanced by the hormone. Activity was promoted in extracts from wheat half-grains, but inhibited in extracts from the two barley cultivars. Whereas monophenolase activity was detected primarily in extracts of the wheat aleurone layer, it was only detectable in the barley endosperm extracts. The inhibitory effect of GA3 on monophenolase activity in the barley cultivars is discussed.