Expression of lipoxygenase was studied in whole developing barley grains from 5 days after flowering (DAF) to full maturity. Lipoxygenase showed two distinct peaks of activity. The first peak of activity occurred in the early stages of grain development from 5 until 20 DAF, whereas the second peak occurred at the later stages, from 30 DAF to the full grain maturity. The temporal expression of both LOX-1 and LOX-2 was studied at the protein and mRNA levels using specific probes and by analyzing the hydroperoxides formed during grain development. These experiments showed that a LOX-2-type isoform is present at early stages of development located in the envelopes of the grain whereas a LOX-1 isoform accumulates in the embryo in the late stages of development. Additionally, the spatial and temporal pattern of hydroperoxide- metabolizing activity was shown to be comparable to that of lipoxygenase activity. We demonstrate that two lipoxygenases appear during barley grain development, differing in their spatial and temporal expression. These two enzymatic pathways give rise to a distinct set of end-products indicating that the lipoxygenases may play different physiological functions during grain development.