Fourteen norditerpenoid alkaloids present in larkspur (Delphinium) species associated with cattle poisoning on grazing land in the western United States have been toxicologically assessed in a mouse bioassay. Toxicity data for these alkaloids have established the tertiary nitrogen atom and anthranilic acid esterification as important structural features necessary to impart toxicity to lycoctonine-type norditerpenoid alkaloids. Variation in C-14 functionality of the toxic alkaloids is also a factor that influences toxicity in these compounds. The relationship of the structure-activity information of this study to previous in vitro neuromuscular studies is discussed.