1-(6-Methoxy-4-quinolyl)-3-(3-vinyl-4-piperidyl)-1-propanone (viquidil, Desclidium) when administered parenterally was a potent inhibitor of thrombus formation in the microvasculature of the hamster cheek pouch. The drug was active at a minimum dose level of approximately 2.5 X 10(-4) mg/kg and appeared to be more potent than papaverine in the same test system. Papaverine was shown to be toxic at higher dose levels with intermittent vasodilatation and haemorrhage. Viquidil, however, appeared to be approximately twice as potent as papaverine and was apparently without toxic side-effects.