Sixty subjects classified as either Type A or Type B interacted in pairs by pressing buttons which transmitted messages through a television screen while the heart rate and digital blood volume pulse were computer-monitored. Partners could cooperate, compete, punish, reward, or withdraw on each interaction and could send 1 of 55 messages communicating feelings, requests, and behavioral intentions between interactions. Interactions and communications between Type A subjects were strikingly different from Type B subjects. Type A's were noticeably more aggressive competitive. Type A dyads also exhibited larger digital vasomotor responses than Type B dyads. When Type A subjects and Type B subjects interacted with each other, rather than with a same-type partner, the differences between them in behavior and vasomotor response largely disappeared.