Prlor work has shown that head-out water immersion (WI) produces a prompt central hypervolemia, a natriuresis, and a diuresis. To assess If cardiac chamber enlargement modulates these effects, we measured cardiac size, shape, wall motion, and cardiac displacement via serial underwater two-dimensional echocardiography. Six normal volunteers underwent 2.5 hours of WI to the neck, seated, at 34.5° C. Recovery was 30 minutes. The size of both atria increased significantly but transiently during the study (p < 0.0001) for left atrium, and p = 0.0020 for right atrium). Both atria returned to baseline size during WI. Moreover, for left atrium there was a small overshoot in recovery. Neither left ventricular nor right ventricular dimensions nor ejection fraction changed significantly. Also, no shape changes were detected, although WI was associated with upward and lateral displacement of the acoustic windows. Correlation coefficlents (r) for left atrial, or right atrial size vs urinary excretion of sodium or urine volume size ranged from 0.05 to 0.36. These results, in sum, suggest that strong compensatory mechanisms are counteracting the effect of WI on distended cardiac receptors, and that cardiac receptor activation alone does not constitute the afferent limb of the reflex mediating the renal effects of head-out WI.