Larkspur plants contain numerous norditerpenoid alkaloids, which include the 7,8-methylenedioxylycoctonine (MDL)-type alkaloids and the N-(methylsuccinimido)anthranoyllycoctonine (MSAL)-type alkaloids. The MSAL-type alkaloids are generally much more toxic (typically >20 times). Toxicity of many tall larkspurs, such as Delphinium barbeyi, has been attributed to its large concentration of MSAL-type alkaloids, including methyllycaconitine (MLA). However, the norditerpenoid alkaloids found in the greatest concentrations in most D. barbeyi populations are either deltaline or 14-O-acetyldictyocarpine (14-OAD), both less toxic MDL-type alkaloids. Although the individual toxicities of MLA, 14-OAD, and deltaline have been determined, the impact (additive or antagonistic) that large concentrations of deltaline or 14-OAD in the plant have on the toxicity of MLA is unknown. Consequently, the effect of MDL-type alkaloids on the toxicity of MLA was compared by using median lethal dose (LD(50)) and toxicokinetic profiles of the brainand muscle from mice receiving i.v. administration of these alkaloids, individually or in combination, at ratios of 1:1, 1:5, and 1:25 MLA to MDL-type alkaloids. The LD(50) for MLA alone was 4.4 +/- 0.7 mg/kg of BW, whereas the coadministration of MLA and deltaline at 1:1, 1:5, and 1:25 resulted in an LD(50) of 2.7, 2.5, and 1.9 mg/kg of BW, respectively. Similarly, the coadministration of MLA and 14-OAD at 1:1, 1:5, and 1:25 resulted in an LD(50) of 3.1, 2.2, and 1.5 mg/kg of BW, respectively. Coadministration of mixtures did not result in increased MLA bioavailability or alterations in clearance from the brain and muscle. Consequently, the increased toxicity of the mixtures was not a result of increased MLA bioavailability (based on the maximum concentrations observed) or alterations in MLA clearance from the brain and muscle, because these were unchanged. These results demonstrate that MDL-type alkaloids have an additive effect on MLA toxicity in mice and may also play a role in the overall toxicity of tall larkspur plants in cattle.