Experimental studies on small blood vessels of the rat mesentery were performed in order to optimize current clinical argon laser coagulation technique of ocular neovascularization. The vessel effects were observed by means of vital microscopy and histological studies were done on coagulated vessels. Furthermore systematic variation of the coagulation parameters was done. The main results were as follows: Graded effects such as reversible vasoconstriction, endovascular thrombosis and complete separation of the vessel wall could be achieved with the argon laser on small vessels in surroundings of relatively low absorption. Short duration vessel closure resulted from endovascular thrombosis alone or in conjunction with downstream vasoconstrictions. Destroyed erythrocytes adhering to the vessel wall played a major role in endovascular thrombosis. Subendothelial structures were of minor importance. Longer lasting vessel closure could be achieved by summation of the effects of single exposures. Low risk vessel coagulation is only possible at exposure times of less than 20 msec. Currently available clinical argon lasers do not deliver the high laser powers needed for this purpose.