1. Experimental neurotropic virus infections previously shown to be altered by ether anesthesia are caused by viruses destroyed in vitro by anesthetic ether; this group includes the viruses of Eastern equine encephalomyelitis, Western equine encephalomyelitis, and St. Louis encephalitis. 2. Experimental neurotropic virus infections which were not altered by ether anesthesia are caused by viruses which are refractory to the in vitro virucidal activity of even large amounts of anesthetic ether; this group includes the viruses of poliomyelitis (Lansing) and rabies. 3. Quantitative studies of the in vitro virucidal activity of ether indicate that concentrations of this anesthetic within the range found in central nervous system tissues of anesthetized animals possess no virucidal activity. 4. The lowest concentration of ether possessing significant virucidal capacity is more than fifteen times the maximum concentration of the anesthetic tolerated by the experimental animal. 5. Concentrations of ether 50 to 100 times the maximum amount tolerated by the anesthetized animal are capable of destroying large amounts of susceptible viruses, the average lethal dose (LD(50)) being reduced more than 5 log units. 6. On the basis of the studies presented in this report, it cannot be concluded that direct virucidal activity of ether is not the underlying mechanism of the inhibition by anesthesia of certain experimental neurotropic virus infections. Indirect inhibition of the virus by the anesthetic through an alteration in the metabolism of either the host cell or the host animal as a whole appears at this point to be a more likely possibility.