Technical Abstract: A rapid quantitative electrospray mass spectrometry method was developed for the screening of larkspur (Delphinium spp.) plant material for toxic norditerpene alkaloids. The method was calibrated for two standard alkaloids, methyllycaconitine (1) and deltaline (2), with a recovery of 92% and relative standard deviations of 6.0% and 8.1% for the two alkaloids. Thirty-three samples of plains larkspur, Delphinium geyeri, were analyzed Methyllycaconitine (1) concentration was 0.27% + 0.08% during a one-month period in 1997 establishing the relative risk of poisoning from the plant to be low. The method was also applied to the trace analysis (<1 ppm) of 1 in serum samples from sheep dosed with different levels of the alkaloid. Electrospray ionization combined with sequential tandem mass spectrometry and HPLC coupled to atmospheric pressure chemical ionization (APCI) mass spectrometry were used to detect and tentatively identify three new norditerpenoid alkaloids from Delphinium nuttallianum [bearline (6), 14- acetylbearline (7), 16-deacetylgeyerline (8)]. The tentative structure of the new alkaloids was predicted from the tandem mass spectra fragmentation patterns and assigning the substitution pattern for methoxy and acetyl groups at the C-14 and C-16 carbons.