Senecio riddellii is a suffruticose perennial containing hepatotoxic pyrrolizidine alkaloids (PAs) like riddelliine, which causes chronic liver disease in range animals. To identify periods and conditions of greatest risk to animals from PA-containing Senecio species on Western rangelands, a three-year survey of PA content across multiple sites was conducted, revealing S. riddellii as the highest PA producer (mean 6.42% dry leaf weight). A subsequent five-year (1979–1983) survey of a site near Woodward, OK, found exceptionally high PA levels in S. riddellii: maximum total PA (free base + N-oxide) in air-dried leaves exceeded 10% annually, with a peak of 17.99% (1980), the highest recorded for any alkaloid in any plant part (surpassing the previous 9.28% in Crotalaria retusa seeds). Peak PA production correlated with the mature pre-flower-bud stage, and free base/N-oxide proportions were constant (mean 14%/86%). These levels raise questions about PA roles in plants; potential factors include infertile sandy soil and 1980’s high temperatures/low rainfall, though no grazing pressure was observed. The 17.99% PA content in leaves (typically alkaloids are highest in seeds/fruits) is unprecedented, exceeding levels in medicinally cultivated alkaloid plants (rarely >7%).