AbstractIntrathoracic stretch receptors regulate adjustments of the vasculature to gravitational changes and influence urinary water and solute excretion. Few reports of pathologic states involving interruption of these regulatory mechanisms have appeared. Two patients with orthostatic hypotension related to advanced intrathoracic carcinoma were studied, utilizing tilt‐table examinations and immersion of the entire body in water to test the function of their intrathoracic baroreceptor reflex arcs. Both patients showed abnormalities of antidiuretic hormone level and sodium excretion as compared with normal controls. This suggests that total immersion is a safe and convenient test of the low‐pressure baroreceptor system in patients with suspected dysfunction. Three patients are also reported whose charts were reviewed posthumously. Although they were not tested in the laboratory, their clinical data suggest that they too had been suffering from an interference with the transmission of impulses from the intrathoracic recept