Eleven norditerpenoid alkaloids found in three tall larkspur species (Delphinium barbeyi, Delphinium occidentale, Delphinium glaucescens) were evaluated as mammalian toxins in a mouse bioassay. Two methylsuccinimido anthranoyllycoctonine (MSAL) norditerpenoid alkaloids (methyllycaconitine and 14-deacetylnudicauline) were determined to have equivalent high toxicity. A third MSAL norditerpenoid, barbinine, was 5 times less toxic. Other norditerpenoid alkaloids tested were found to be significantly less toxic than the MSAL alkaloids. The chemical phenology of the alkaloids in the three larkspurs and the effect of collection site on alkaloid occurrence in D. barbeyi were determined.