Abstract: An interactive injection system for use with a catheter of the type adapted for insertion into a patient's vascular system for thermodilution measurements and the like includes a chamber for storing liquid injectate, means for connecting the chamber to the catheter lumen, and a drive for operating on the chamber to flow liquid injectate from the chamber into the lumen for discharge as a bolus from the lumen into the vascular system of a patient. In preferred embodiments, a temperature sensing probe is connected between the injection chamber and the catheter for monitoring the temperature of injectate as it is introduced into the catheter.