In order to clarify the nature of the physiological adjustment for the production of whispering, physiological studies were conducted on the glottal and supralaryngeal adjustments during whispering. The results indicate that for the production of whispering there is a necessary relationship between glottal adjustment and supralaryngeal adjustment in terms of the coordination between posterior cricoarytenoid muscle (PCA) and thyropharyngeus muscle (TP). Also, there might be a function switch in our brain which enables the speaking mode to change from ordinary to whispering, using the activation of PCA and TP, as a result of human evolution.