The aim of the present work was to determine an influence of grain aphid, Sitobion avenae F. feeding on ornithine decarboxylase (ODC) activity within tissues of winter triticale. Experiments were carried out on aerial parts and roots of seedlings of two triticale cultivars: Tornado - more susceptible to S. avenae and Witon - less susceptible. The obtained results showed that ODC activity in shoots of cv. Witon decreased during the first week of grain aphid feeding and increased after 2 weeks. However, in the roots of this cultivar the enzyme activity decreased after 48 h and increased after 2 weeks. When susceptible cv. Tornado was studied the grain aphid attack reduced the ODC activity after 24 h of feeding. Moreover, the grain aphid attack reduced the ODC activity during the first week and increased after 2 weeks in the shoots and roots of cv. Tornado. The importance of the ODC activity and polyamines in stress reaction and defense mechanisms of winter triticale to S. avenae is discussed.