The effects of polystyrene cerebral emboli of the order of 60 to 120μ in the monkey have been studied by serial conventional angiography (indirect effects) and photographically enlarged industrial filming (direct effects). Findings were: diffuse or locally poor capillary phase, mixed arteriovenous phases, general or locally poor venous phase, arterial dilatation and stasis within vessels over 500μ in diameter. A suggestion of leptomeningeal immediate collaterals and definite spasm of small arteries could be seen. The emboli generally could be seen only with photographically enlarged industrial arteriograms. Papaverine hydrochloride caused vasodilatation of arteries less than 300μ, with no discernible effects on larger arteries. The poor capillary and venous phases in the embolized animal were improved with papaverine. Tolazoline hydrochloride administered intra-arterially caused marked vasoconstriction of vessels smaller than 300μ and diminution in the capillary and venous phases, and was uniformly associated with mortality of the animal in 30 minutes.