Selected inotropic drugs were studied in an isolated right ventricle preparation in which the work load and heart rate could be controlled. Records of ventricular pressure and volume were made. Rate of ventricular relaxation was estimated, and mechanical impedance to inflow was calculated as the ratio of intraventricular pressure to inflow. The negatively inotropic drugs slowed relaxation and increased impedance. Strophanthin-K reversed these effects. Adrenalin decreased impedance. It is concluded that, during the filling period, inotropic drugs alter those physical properties of ventricle wall which contribute to mechanical impedance.