Sophisticated electrophysiologic monitoring approaches were used to: 1) show greater sensitivity in nonhostile young men to T-wave attenuation effects of isoproterenol infusion following vagal blockade; 2) show that vagal enhancement reduced and shortened the T-wave attenuation effects of isoproterenol infusion more in hostile young men; 3) evaluate these effects of vagal blockage and enhancement in middle-aged men and in young and middle-aged women; and 4) relate the T-wave effects in these studies to other measures of vagal tone and other biobehavioral mechanisms of coronary-prone behavior.