The authors investigated blood pressure changes (Erlanger method) and inspiration-expiration ratios under conditions of experimental truth and deception. They were interested in securing records under some standard and repeatable conditions and in application of the method to situations of low stimulation value." Both blood pressure and respiratory changes may be considered as of diagnostic value in detection of deception, but there is real need of a standardization of the routine and technique and of a more critical attitude. The increase in the inspiration-expiration ratio after falsehood as compared to truth may be part of the mechanism of the "gasp of surprise.".