Little information is known concerning hemodynamic parameters of bypass grafts after open heart surgery during the postoperative period. As there is no easy to use bedside-method for monitoring, we developed a new implantable Doppler probe to measure blood flow velocity with a 20 MHz pulsed Doppler system. The accuracy of this method was proved in a circulatory model and in animal studies. Our conclusion is that the implantable pulsed Doppler system is a sensitive method for monitoring small vessel hemodynamics.