BACKGROUND: Narrow-leaved lupins (NLL, Lupinus angustifolius L.) is recognized as a climate-resilient protein crop but its usein food and feed is frequently limited by toxic quinolizidine alkaloids (QAs). The effect of intercropping with spring oat (Avenasativa L.) on grain QA content has not yet been quantified. RESULTS: In a 2-year field experiment, three NLL varieties (Lunabor, Probor, and Jowisz), grown as pure stands and in nine mixtureswith the oat varieties Bison, Lion, and Troll were compared. Mixed cropping increased total grain QAs by between 16% and 46% rel-ative to the respective pure stands. Absolute increases reached +168 mg kg−1 in Lunabor and +128 mg kg−1 in Probor, whereasJowisz increased by only +76 mg kg−1. Among the mixtures, Jowisz–Bison exhibited the smallest increase (16%) and the lowest finalQA content, whereas Lunabor–Troll showed the highest content. In mixed stands, both Lunabor and Probor exceeded the 500 mgkg−1 threshold, whereas Jowisz remained below this threshold. Profiles of the seven major QAs remained constant, with the excep-tion of the 13-hydroxylupanine to lupanine ratio, which increased in the mixture. Year effects were not observed. Conclusion: Intercropping NLL with oat elevates the grain QA content to levels of toxicological relevance. The extent is variety-interaction dependent, presumably due to oat allelopathy. The evidence points to an indirect stress mechanism: allelopathiccues from the oat crop place NLLs under physiological stress, which in turn stimulates NLL to accumulate additional QAs inthe grain. Additional mixed cropping experiments and breeding against QA accumulation in NLL grains should be pursuedto understand and alleviate this issue.