Forearm arterial flow was measured in 22 healthy first-time blood donors during a 300-ml. blood letting and during the subsequent recovery. Blood pressure (BP) was also taken simultaneously and forearm peripheral resistance calculated. Following a transient BP and flow increase due to tachycardia related to needle insertion, both systolic BP and flow progressively and significantly decreased, while resistance increased. In a further 22 sex- and aged-matched highly hypnotizable subjects, blood donation was simulated by means of verbal hypnotic suggestions. The BP, flow and resistance curves were similar to those obtained with the real blood letting, without any between-subject difference or group/time interaction. Mere hypnosis without suggestion of phlebotomy and the simple bed resting did not produce any effect. These results indicate that the hemodynamic changes observed during and after a blood loss are partly due to mental involvement rather than merely to the hydraulic effects of the removal of blood.