The toxicity of methoxyacetone was compared to that of methyl n-butyl ketone and a series of four other aliphatic ketones (diethyl ketone, ethyl n- buty1 ketone, methyl isobutyl ketone-, methyl tert-butyl ketone) with particular reference to neurotoxicity. Preliminary studies were performed to establish toxic dosage levels of the ketones, which were administered in drinking water for seven days at concentrations of 0.5% and 1% (estimated to be equivalent to approx. 0.3 to 0.9 gm/kg/day). Reduced food and water intake and low body weight gains probably reflected rejection of the water because of taste. The concentrations used were at or near maximum water solubility of most of the ketones, methoxyacetone being the most notable exception. Methoxyacetone produced no toxic effects at these concentrations or at 2%, but 6% and 20% solutions of this material for 7 days produced dosage-related effects similar to those of the other ketones.