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1. Fluid transport rate and oxygen consumption (Q_O2) were studied in rabbit gall-bladder preparations in vitro exposed on both sides to identical Ringer solutions with NaCl concentrations (and osmolarities) varying from 70 to 140 m-equiv Na⁺/l.).

2. The time sequence of acute effects on transport rate resulting from sudden changes in the NaCl concentration of the bathing solutions indicated that, (a) as a primary effect, fluid volume transfer rate remained unaffected whereas Na transport rate changed abruptly in direct proportion to the Na concentration of the bathing media; (b) a secondary, delayed and partly reversible depression of fluid transfer rate following elevation of the NaCl concentration was observed only when the rate of transport was relatively high initially.

3. A fixed, and highly significant, linear relationship between changes in transport-linked oxygen consumption (Q_O2) and measured net fluid volume transport (T_vol) was found independent of the NaCl concentration of the bathing media, dQ_O2/dT_vol being 0·22 ± 11% and 0·25 ± 8% in bladders incubated in solutions containing 140 and 70 m-equiv Na⁺/l. respectively.

4. Oxygen consumption per equiv of Na⁺ (calculated) transported varied in inverse proportion to the Na concentration of the bathing media, dQ_O2/dT_Na being 0·0016 ± 11% and 0·0036 ± 8% in `140 R' and `70 R' solutions, respectively.

5. Removal of K from the bathing solutions was followed by a gradual and partly reversible depression of fluid transport rate to a minimum level (about 100 x 10^-4 µl H₂O. min^-1.mg^-1) independent of the initial transport rate.

6. It is concluded that the range of absorption rates of isosmotic fluid from the gall-bladder lumen represents a range of energy requiring capacities for transfer of fluid volume units; the data suggest that the intracellular (cytoplasmic) ion composition, depending on the presence of external K, as well as hormonal action may influence the capacity of the transcellular fluid transport mechanism.

7. A model (a `mechanical volume pump') for transcellular transfer of fluid volume units, allowing for flexible specificity with regard to the actively transported solutes, and requiring the presence of Na⁺ and Cl^-, is proposed.