Bed rest and immersion both lead to a mobilization of extravascular fluid and thus reduce edema. This study compared three treatments for edema in healthy pregnant women in the third trimester: lateral supine bed rest at room temperature, sitting in a bathtub of waist-deep water at 32 +/- 0.5C with legs horizontal, and sitting immersed in shoulder-deep water at 32 +/- 0.5C with legs extended downward. Post-treatment diuresis was selected as the indicator of extravascular fluid mobilization. The mean (+/- SD) diuresis was 105 +/- 48, 161 +/- 155, or 242 +/- 161 mL/hour for bed rest, bathtub, and immersion tank, respectively (P less than .008, tank versus bed rest; P less than .05, tank versus bath). In all treatments, mean arterial pressure (MAP) declined from a baseline value of 88 +/- 9 to 77 +/- 10 mmHg 25 minutes into treatment and 77 +/- 11 mmHg at 50 minutes (both P less than .0001 compared with pre-treatment). Shoulder-deep immersion produced the greatest decline in MAP. Sodium clearance increased from 0.7 to 1.0 mEq/minute in all treatments (P less than .01). Serum sodium, potassium, creatinine, osmolarity, total protein, 6-keto prostaglandin F1 alpha, and plasma volume did not change significantly after the treatments. Serum prolactin declined significantly from 137.8 +/- 44 to 124 +/- 31 ng/mL after treatment; there was no difference among treatments. Immersion appears to be a safe and more rapid method than bed rest to mobilize extravascular fluid during pregnancy.