AbstractAn analysis of the pyrrolizidine alkaloids (PA) of the introduced annual weed,Heliotropium europaeum(Boraginaceae), at two sites in south‐eastern Australia showed that percentage PA in above‐ground parts of the weed differed significantly between sites and declined markedly through the season, from initial levels of>2.2% in the seedling stage to95% of total alkaloid. Major PA constituents — europine, heliotrine, and lasiocarpine — varied significantly between sites and sampling dates but no marked seasonal trend was detected. Lasiocarpine and europine content varied reciprocally in the free base andN‐oxide forms, indicating that these compounds may be readily interconvertible. Correlation of rainfall and average number of leaves per plant with lasiocarpine and europine levels suggested that rainfall, either directly by altering the metabolism of existing leaves or indirectly through plant growth, initiated large short‐term shifts in PA composition ofH. europaeumwithout corresponding changes in total PA concentration.At both sites, sheep consumedH. europaeum, reducing both plant density and biomass. Biomass differences between exclosure and access plots ofH. europaeumwere correlated with grazing intensity. Since heliotrine, lasiocarpine and europine differ markedly in their toxic properties, large shifts in their relative concentrations may have important consequences for these and other herbivores. Environmentally induced quality shifts in these secondary compounds may be as important as, or even override, changes in total quantity in causing PA toxicity to gr