A 66-year-old man with infectious and therapeutic toxicities developed ventricular fibrillation after less than 4 seconds of diagnostic carotid sinus pressure. This is illustrated by an electrocardiogram. A 70-year-old woman with posterior infarction of the heart had carotid sinus pressure applied to correct a tachycardia and developed total cardiac standstill. Both patients died. If the application of pressure to the carotid sinus is being considered for either therapeutic or diagnostic purposes, the authors urge that it be unilateral only, limited to 5-second periods, monitored by auscultation or preferably by electrocardiography, used with special care in elderly patients receiving cardioactive drugs for known cardiovascular disease, and avoided altogether if the initial systolic pressure is below 100 mm. Hg.