Infusions of Chenopodium ambrosioides (L.) have been used for centuries in the Americas as a popular remedy against intestinal worm infections. The essential oil of Chenopodium ambrosioides contains high levels of ascaridole, which is a potent anthelmintic, but which has also been responsible for human fatalities, leading to its disuse. Almost 90% of the nematocidal activity of Chenopodium ambrosioides infusions was due to a hydrophilic component different from ascaridole. Synthetic ascaridole and the ascaridole from infusions, extracted into hexane, caused a reduction of carbachol-induced contractions in rat gastrointestinal smooth muscle at concentrations required to kill Caenorhabditis elegans (L.). The herbal infusion and the ascaridole-free hexane-extracted aqueous residue of the above infusion, at nematocidal concentrations, had no detectable effect on smooth muscle contraction in the above system. It would appear that the traditional form of usage of Chenopodium ambrosioides infusions as a vermifuge is safer than the use of the herb's essential oil.