Patients undergoing elective cardioversion for treatment of arrhythmias were premedicated with diazepam. The dose was individualized to achieve a degree of central nervous system CNS) depression characterized by response to painful but not vocal stimulation. Promptly following each cardioversion, blood was drawn and the plasma diazepam concentration was measured by gas chromatography. The plasma levels varied as widely as the diazepam doses so that there was no fixed plasma level of diazepam associated with the degree of CNS depression produced in these patients. However, both the dose of diazepam and the resulting plasma level were inversely correlated to age, indicating that age is a critical factor in the use of diazepam for cardioversion premedication; elderly are more sensitive to the depressant effects of this drug than the young.