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 < .008, tank versus bed rest; P < .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 < .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 < .01). Serum sodium, potassium, creatinine, osmolarity, total protein, 6-keto prostaglandin F1α, 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.