The aim of our study was to quantify changes in the activity of tyrosine decarboxylase (TYDC) within tissues of two cultivars of winter triticale: Triticosecale (Wittm. ex A. Camus), caused by grain aphid, and Sitobion avenae (F.). Obtained results show that Witon cv was less acceptable by S. avenae than Tornado cv and that antibiosis was an important part of the acceptability. The grain aphid feeding on aerial parts of more acceptable Tornado cv caused fluctuations in TYDC activity instead of on less acceptable Witon cv, and a constant increase in enzyme activity was observed. Changes in the TYDC activity within the triticale tissues had a systemic character, since they were present not only in aerial parts directly attacked by the pest, but also within root tissues. It suggests that induction of the enzyme activity is related to an antibiotic response of the triticale towards the grain aphid.