The pyrrolizidine alkaloid and corresponding N-oxide contents of small samples ofSenecio longilobus, S. riddellii, S. jacobaea, and S. vulgaris have been measured using nuclear magnetic resonance spectroscopy. The percentages of the individual alkaloids seneciphylline, senecionine, riddelliine, and retrorsine in the total alkaloid mixture have also been determined from the same NMR spectra. Exceptionally high total alkaloid contents were found for S. longilobus and S. riddellii relative to most Senecio sp. The technique provides a rapid, facile method for examination of plant samples in order to evaluate the hepatotoxicity hazard to animals and humans and for regulation of dose rate of pyrrolizidine alkaloids in animal feeding experiments. © 1979, American Chemical Society. All rights reserved.