The phenomena of general anesthesia had scarcely been appreciated before the corollary arose: Does narcosis harm the animal economy, and, if so, in what particulars? A number of physiologic and clinical studies have been made by different investigators to determine whether the anesthetics ordinarily employed are harmful. The present study may be grouped with these researches and was undertaken at the suggestion of Dr. A. D. Bevan with the hope of defining as accurately as possible the blood changes incident to nitrous oxid anesthesia. The increasing use of this gas as an anesthetic in major surgical operations, as well as the increasing time of anesthesia, were factors which prompted the investigation. In particular, we attempted to answer the following questions: 1. Does nitrous oxid reduce the hemoglobin and thereby cause anemia? 2. Does it cause hemolysis? 3. Does it increase the coagulability of the blood and thereby increase the danger