Changes in content of cyanogenic glycosides and in cyanogenesis potential within the bird cherry leaves during occurrence of the bird cherry-oat aphid, Rhopalosiphum padi L. and while its spring host-plants alternation have been studied. The highest content of the cyanogenic glycosides and the highest cyanogenesis potential was found in the youngest leaves of the primary host during occurrence of the first fundatrices. When the aphid population started to build up, a decrease in content of the cyanogenic glycoside and in the cyanogenesis potential was observed. Finally, when the winged migrants began to fly off from the primary host onto cereals, pretty low amount of the plant xenobiotics was recorded within the bird cherry leaves. Possible role of the cyanogenesis in host alternation of the bird cherry-oat aphid is discussed.