1. The usual predominance of the intensity of the first heart sound over that of the second heart sound frequently may be reversed during anaesthesia produced by Fluothane, the intensity of the second heart sound becoming greater relative to the first heart sound. 2. Four electroencephalographic patterns were discerned during anaesthesia produced by Fluothane, and a good correlation could be shown to exist between these patterns and the concentration of the inhaled vapour of Fluothane in most instances. 3. No constant correlation could be defined between the changes in the heart sounds and either the concentration of the inhaled vapour of Fluothane or the electroencephalographic patterns produced by this drug. 4. Preliminary attempts to employ the changes in heart sounds during Fluothane anaesthesia as a guide to anaesthetic depth have been unsuccessful.