Since acetaldehyde boils at 21[degree] C and, in spite of its pungent ethereal and almost suffocating odor, may be respired by animals, it was decided to investigate its effect when administered by inhalation. Acetaldehyde, freshly distilled from paraldehyde in the presence of sulftiric acid and water, was used. Preliminary experiments in a number of rats, cats and dogs revealed that these animals can be easily anesthetized with pure or aqueous solns. of acetaldehyde and recover quickly once the admin. of the aldehyde is terminated. In the 2d series of expts., dogs were anesthetized with undiluted or 30% aqueous solns. of acetaldehyde applied by means of a cone or a towel wrapped around the nostrils and mouth. Within 5 min. or less the animals were completely relaxed. The dogs were then operated on and the trachea cannulated and the common carotid artery exposed and cannulated for recording blood pressure. Unknown amts. of acetaldehyde produced anesthesia upon inhalation without respiratory or circulatory depression. Acetaldehyde used alone or in combination with other anesthetic agents stimulated respiration and raised the blood pressure. Acetaldehyde relaxed the bronchial musculature. It is to be remembered that the inhalation of concentrated acetaldehyde vapors is likely to produce cardiac, vascular and respiratory toxic effects and that in unanesthetized humans such concentrated vapors are probably irrespirable.