(-)-Hyoscyamine-d3 is intended for use as an internal standard for the quantification of (-)-hyoscyamine (Item No. 25644) by GC- or LC-MS. (-)-Hyoscyamine is a tropane alkaloid, a muscarinic receptor antagonist, and an isomer of atropine (Item No. 12008).{43484,43485} (-)-Hyoscyamine inhibits the arterial depressor response induced by the muscarinic agonist methacholine in dogs.{43484} It is more effective than atropine at inhibiting EEG arousal in dogs following sciatic nerve stimulation when administered at doses of 0.5 and 1 mg/kg. (-)-Hyoscyamine induces convulsions in and is toxic to mice (LD50 = 95 mg/kg).{43485}