Several human fatalities have recently been reported for which the intravenous injection of a mercurial diuretic was believed to be the causative agent.1 It therefore seemed desirable to make a comparison of toxicity of the mercurial diuretics by this route and to study the influence of the drugs commonly used in the treatment of heart disease on this toxicity. The drugs used for the comparison were esidrone, mercupurin and salyrgan-theophylline and the three parent mercurials, esidrone without theophylline, mercurin and salyrgan. PROCEDURE AND RESULTS Comparative Acute Lethal Dose.— The cat was selected as the experimental animal. The drugs were injected without dilution so that the concentration of mercury was approximately 40 mg. per cubic centimeter in every case. Injections were given into the femoral vein by means of a micro buret connected to a 19 gage needle with a short piece of rubber catheter. No anesthesia was used and