Introduction The concept of the sedation threshold was introduced by Shagass3-9in 1955, and it has since received considerable attention. The sedation threshold is defined by Shagass as the amount of amobarbital sodium, expressed in milligrams per kilogram of body weight, required to produce an inflection point in the amplitude curve which appears when the frontal 15- to 30-cps electroencephalographic activity is plotted against the amount of amobarbital sodium given. Nearly all such curves contain an inflection point at which the curve, after a steep rise, tends to plateau or fall. The appearance of this inflection point is often coincident with onset of slurred speech, and the latter sign has, therefore, been used as an indication that a sufficient amount of amobarbital has been given. Shagass found that the sedation threshold was a relatively constant biological characteristic of the normal subject, as indicated by