Anticholinesterase medications (anti-ChEs) play a significant role in the diagnosis and treatment of myasthenia gravis (MG). The primary effect on the heart produced by a surfeit of ACh is bradyarrhythmias with consequent fall in cardiac output and hypotension; yet, adverse cardiac reactions to these agents have been reported relatively infrequently. The authors describe 12 patients with MG from a pool of more than 1,000 who suffered hypotensive episodes related to use of anti-ChEs. The 12 patients (seven male, five female) had a mean age of 62.6 years; of these, eight adverse reactions occurred after edrophonium, two after neostigmine, and two after pyridostigmine. Seven patients had a recent increase in anti-ChEs and none had a decrease in dosage. Nine patients suffered either from severe sinus bradycardia, (20 beats/min), junctional bradycardia, or complete AV dissociation. Two patients had paradoxic sinus tachycardia and all had syncopal or near-syncopal episodes. Evidence for cholinergic stimulation of other organs was generally lacking. No recurrence appeared with reduction of the dose of anti-ChEs or discontinuation of the drug. The authors believe that these agents should be given with caution to patients with inflammatory, infiltrative, or degenerative disease of the conduction systems, patients being treated with digitalis, calcium-channel antagonists or beta blockers, patients with myocardial ischemia, and elderly patients. Appropriate resuscitative equipment should be readily available.