The continuous instillation of plasmin (Thrombolysin) directly into the main pulmonary artery of greyhounds, using one half of the systemic doses, was found to be more effective in accelerating or enhancing in vivo lysis of artificially produced pulmonary emboli than systemic intravenous infusion. Animals which received direct pulmonary arterial instillations of Thrombolysin showed remarkably few macroemboli. By light microscopy, these clots showed zones of lysis mainly in the central portions of the clots. We suggest that the flow-guided catheterization of the main pulmonary artery used in this study on greyhounds can be applied to critically ill human subjects, as shown by earlier studies (Magtira, Winkler, Duprey, and Baum, 1966). Continuous infusions of thrombolytic agents directly into the main pulmonary artery might then offer an effective therapy for human pulmonary embolism, especially during that critical period following a massive or sUbmassive embolic episode.