In previous papers the authors (Kerckhoffs et al. 1966; Penning et al. 1966) reported experimental evaluation of emulsified iophendylate (Pantopaque, Ethiodan) in the cranial subarachnoid space. It was shown that the emulsion had a toxic effect on the cranial nervous system of rabbits, manifesting itself in the form of convulsions, paresis, and death. Nonemulsified iophendylate, however, proved to have no toxic effect. The toxicity was not caused by the emulsifying agents, but appeared to be directly related to the degree of dispersion of the emulsion. It was concluded that iophendylate itself possessed latent toxic properties which were enhanced by the emulsification. Previous investigations with emulsified iophendylate were reported by Pirkey et al. (1950), Jaeger (1950), Nosik (1951), Portera-Sanchez et al. (1964), Fisher (1965), Kunz et al. (1965), Maupin et al. (1966), and others. These investigators all agree on the good radiographic qualities of the emulsion in comparison with the nonemulsified iophendylate. With respect to the toxic properties opinions diverge, but most investigators conclude that the toxicity of the contrast agent is increased significantly after emulsification. Because of the good radiographic qualities of emulsified radiopaque oils, especially in ventriculography and myelography (Nosik, Matsubara and Nomura, Portera, Obrador, Reyes et al.), the toxicity of emulsified iophendylate was a disappointing finding, because it obviates clinical application. We therefore planned to evaluate the toxic effect of emulsions of other oily radiopaque materials, hoping to find a lower or absent toxicity which would justify its use in clinical practice. Emulsions of iodized oil with sodium lauryl sulfate as emulsifier were used by Matsubara and Nomura (1960) in ventric-ulography in man. After the radiographic procedures, the emulsion was withdrawn from the ventricles. Side-reactions were temporary fever, appearing after several hours, slight headache, and, infrequently, nausea and vomiting. Experimental use of emulsified iodized oil was also reported by Jaeger. He prepared emulsions with acacia and blood serum. Injection into the lumbar subarachnoid space, the grand cistern, and the cerebral ventricles of dogs caused the death of almost all animals within twenty-four hours to two weeks. Moreover, after injection via the lumbar route, paralysis was seen. A more recent type of oily radiopaque is Ethiodol (Lipiodol Ultra Fluid, Iodetryl, the ethyl ester of the iodized fatty acids of poppy-seed oil), a product of the research toward increasing the fluidity of classical Lipiodol. Fisher demonstrated that lumbar injections of Ethiodol emulsions, prepared with lecithin, into the subarachnoid space in cats were well tolerated. The results showed a satisfactory contrast, no demonstrable toxicity, and essentially complete absorption after myelography.