G. A. Levvy in 1940 (Biochem. J., 34, 73) published from the Department of Pharmacology, University of Edinburgh, a new method for the estimation of barbiturates in blood, claiming that his technique gave results accurate to a 20 per cent. error or less for quantities of 1.0 mgm. or more in 20 c.c. of blood. After studying this method at first hand we have carried out a number of estimations in cases undergoing prolonged narcosis with somnifaine, and studied the blood levels of barbiturates in patients under medication with soluble barbitone and phenobarbitone, a total of 200 estimations having been made.