The enzymes ฮฑ-amylase (ฮฑ-1, 4-glucan 4-glucanohydrolase, 3.2.1.1), ฮฒ-amylase (ฮฑ-1,4-glucan maltohydrolase, 3.2.1.2) and phosphorylase (ฮฑ-1,4-glucan: orthophosphate glucosyltransferase, 2.4.1.1) were assayed in whole grains of barley throughout the maturation period. ฮฑ-amylase and phosphorylase had peaks of activity between 25 and 30 days after anthesis. On the other hand the activity of ฮฒ-amylase in both the available and latent forms reached a maximum value at 35 days after anthesis which did not decrease thereafter. ฮฒ-amylase activity was also assayed throughout development in the endosperm, aleurone, testa pericarp and embryo. Latent ฮฒ-amylase reached a constant maximum value in endosperm at 35 days but available ฮฒ-amylase reached a peak of activity at 25 days and then declined to zero at 45 days. Only latent ฮฒ-amylase was associated with the aleurone layer and activity rose to a maximum value at 35 days. The testa pericarp had mainly latent ฮฒ-amylase whose activity fell from an early maximum at 21 days to zero at 35 days. No hydrolytic activity was associated with the embryo. The phosphorylase activity was low and mainly associated with the endosperm fraction.