To help establish the dynamics of the human body the mechanical impedance was measured as two subjects were exposed to vertical sinusoidal motion at frequencies from 1 to 20 cycles per second. The impedance in the supine, lateral, and standing subject positions and its variation due to voluntary change in muscle tone and due to padding the support were determined. In all tests the frequency interval from 4 to 7 1/2 cycles per second was found to contain the initial whole body resonance. Nonlinearity of the response, as established by impedance dependence upon shaketable acceleration level, was observed by altering the peak shaketable acceleration from 0.2 to 0.35 to 0.5 g. The degree of nonlinearity was found to be dependent upon subject position as well as subject physiological differences. Changes in support padding and muscle tone produced substantial alterations in impedance but little variation of resonant frequencies.