Senecio oxyphyllus DC. occurs in Rio Grande do Sul, southern Brazil. Pyrrolizidine alkaloids (0.4% dry weight) were isolated and identified from it, with retrorsine as the major alkaloid and ligularizine as a minor component. The plant is a major cause of livestock poisoning in Rio Grande do Sul, and its pathology has been studied experimentally in cattle. Retrorsine is known to be toxic to horses, cattle and a variety of laboratory animals and is likely to be the cause of the observed pathology.