A new N-(methylsuccinimido)anthranoyllycoctonine norditerpenoid alkaloid, geyerline, has been isolated and characterized from extracts of the poisonous larkspur Delphinium glaucum. A previously described norditerpenoid alkaloid, grandiflorine, has also been isolated from Delphinium geyeri. Both alkaloids are closely related structurally to the potent neurotoxin methyllycaconitine, established as the primary toxin in many larkspurs poisonous to cattle. Mouse bioassay tests showed grandiflorine to possess toxicity comparable to methyllycaconitine, while its synthetically derived monoacetate, grandiflorine acetate, and geyerline are significantly less toxic.