The transmission to the underlying tissue of externally applied negative pressure (2-50 mmHg) was studied in 15 male volunteers on a segment of the upper arm 8 cm in length enclosed in a clear plastic cylinder sealed hermetically against the skin. Pressure recordings were obtained from the anterior and posterior tissue compartments from sites along the entire tissue segment exposed to negative pressure at depths from the skin surface ranging from 5 to 62 mm. Reduction of pressure in the cylinder caused rapid decline, and cessation of external negative pressure rapid recovery, of tissue pressure. In the steady-state phase of negative tissue pressure, the applied external pressure change was in the great majority of experiments transmitted fully or almost fully to the tissue, regardless of from which position along the tissue segment and from which tissue depth pressure was recorded and regardless of the magnitude of the applied negative pressure. The described findings suggest that a defined reduction of atmospheric pressure leads to a similar alteration of vascular transmural pressure and that the technique of external negative pressure can be used for not only qualitative but also quantitative studies of local circulatory reactions evoked by such transmural pressure changes.