Petasites hybridus (Asteraceae), butter bur, is an ancient medicinal plant with spasmolytic sesquiterpene esters. Two chemotypes, the petasine and the furanopetasine chemotype, occur in Austria. The first one is considered as pharmaceutically useful due to its spasmolytic constituents, but it is restricted to the northern parts of the Alps. This use, however, is impaired by the presence of low amounts of toxic pyrrolizidine alkaloids (PA), mainly senecionine and intergerrimine. PA are usually concentrated in the metabolically active parts of the complex rhizome which are the thickenings just below the leaves. They are also present in flower stalks but are almost absent in leaf buds, the petioles and the leaf blades. The alkaloids showed a great variability within and between populations; the values recorded ranged from less than 2 to 500mgkg(-1) PA, median PA of 77 populations varied from 2 to 191mgkg(-1) in the rhizomes. In nearly 25% of the samples analysed the PA content was below 10mgkg(-1), another 25% had between 10 and 20mgkg(-1) PA. Histograms of PA concentrations in a population often showed a distinct skewness toward lower alkaloid contents. Alkaloid content was independent of sesquiterpene chemotype. The seasonal influence on PA content of rhizomes was little in comparison to the variability within the population or within the rhizome itself. Nevertheless, when comparable rhizome parts within a population were considered, the PA content may remain stable over several years. Although plants totally free of PA could not yet be found, it is possible to select populations low in alkaloids. Several populations of the petasine chemotype containing less than 10mgkg(-1) in the rhizomes could be found in the area investigated.