The study examined changes in lysine decarboxylase (LDC), ornithine decarboxylase (ODC) and tyrosine decarboxylase (TyDC) activity in tissues of pea (Pisum sativum L.) infested by the pea aphid (Acyrthosiphon pisum Harris). The aphid induced increased ODC activity after one day and at two weeks. The effect was clearly systemic. TyDC activity increased after one day and at one week at feeding sites (aerial parts), while LDC activity increased only after one day of infestation and then decreased. Attack by aphids also affected enzyme activity in root tissues not directly damaged by the herbivores. The mechanisms of the response induced by pea aphid infestation in pea are discussed.