1. In a preliminary study, a positive pressure of 25 mmHg applied to the lower body raised right atrial pressure by a mean of 7 mmHg.

2. Sustained application of lower-body positive pressure (LBPP) in six normal adult males increased sodium excretion ([Na]V) from a control level of 126·5 ± 10 μmol/min to 213 ± 21 μmol/min (P = 0·003) and fractional sodium excretion (EfNa) from 0·7 ± 0·1 to 1·2 ± 0·1 (P = 0·001).

3. Urine flow (UF) increased from 0·85 ± 0·07 ml/min to 4·1 ± 0·8 ml/min (P = 0·002), osmolar clearance (Cosm) from 2·6 ± 0·13 ml/min to 4·2 ± 0·4 ml/min (P = 0·003) and free water clearance (CH2O)from −1·75 ± 0·1 ml/min to −0·1 ± 0·01 ml/min (P = 0·001). Creatinine clearance (Ccr) showed no significant change.

4. After dopamine blockade with domperidone, LBPP did not cause a rise in [Na]V or EfNa. However, urine flow, Cosm and CH2O remained significantly above control values, implying persistent suppression of antidiuretic hormone.

5. Dopamine blockade without positive pressure did not affect basal sodium excretion.

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