Abstract The correlation between polyamine content and paraquat resistance in horseweed ( Erigeron canadensis L.) was investigated. Untreated paraquat-resistant and paraquat/atrazine-coresistant plants contained about two to three times higher amounts of polyamines, especially putrescine, compared to sensitive ones. A marked increase of polyamine content was detected after paraquat treatment in each biotype which decreased to the initial level in 6 h. Functional activity (characterized by variable fluorescence) of sensitive plants continuously decreased after paraquat treatment, while resistant biotypes recovered their activities after a transitory inhibition. Changes in polyamine content did not correlate with the changes in functional activity. Exogenously added 10 −4 mol L −1 putrescine combined with 10 −5 mol L −1 paraquat in detached sensitive leaves showed a slight protective effect, while paraquat-resistant and paraquat/atrazine-coresistant leaves remained unaffected. The polyamine level of the cycloheximide-treated or long-term cold-treated plants also changed independently from their functional activity. It can be concluded that the higher polyamine level of the treated resistant leaves and the increase of the polyamine content after paraquat treatment seem to be a general stress response rather than the specific reason for paraquat resistance.