The invention provides a method of delivering a therapeutic agent to the adventitia of a vessel using a catheter-based microsyringe. A therapeutic agent is formed into microparticles, which are dispersed throughout an appropriate liquid carrier to form a therapeutic mixture. A catheter is provided that includes a microsyringe operably attached to an actuator. The microsyringe includes a hollow needle in fluid communication with a therapeutic agent delivery conduit. The catheter is introduced into a target area of a vessel. The actuator is operated to thrust the needle into a wall of the vessel. The therapeutic mixture is supplied to the therapeutic agent delivery conduit and delivered through the conduit to the needle and thereby into the adventitia of the vessel. The actuator is again operated to withdraw the needle from the wall of the vessel and to enclose it within the actuator. The catheter is then removed from the vessel.