The dynamic characteristics of seated humans were measured in a laboratory environment. The seat/occupant system was excited vertically with random vibration. Relevant transfer functions were computed using real time acceleration signals fed to a Fourier Analyzer. The transfer functions describe the seat response, the human response, and the combined response in the frequency range from 2 to 20 Hz. Of possible significance in ride quality studies are the natural modes of vibration which were identified; these include a 3.0 Hz “head-nod” mode, a 3.9 Hz seat vertical mode, a 5.6 Hz human response mode, and a seat “back-slap” mode occurring at 11 Hz.