1. The relationship between osmolal clearance (Cosm) and the reabsorption of solute free water (Tch2o) was examined during mannitol and hypertonic saline diureses in both normocalcaemic and hypercalcaemic rabbits.

2. At high values for Cosm (2–4 ml/min) in normocalcaemic rabbits Tch2o during saline diuresis exceeded that during mannitol diuresis as previously shown.

3. Prolonged hypercalcaemia (72–96 h) reduced Tch2o in both mannitol and saline diureses.

4. Brief hypercalcaemia (18–24 h) abolished the increment in Tch2o normally seen during saline, as compared with mannitol diuresis. This impairment of Tch2o during saline diuresis occurred at a time after the start of hypercalcaemia when Tch2o during mannitol diuresis was still unaffected.

5. The results cannot be attributed to an effect of hypercalcaemia on membrane permeability to water, and are best explained by a reversible limitation of sodium transport in Henle's loop. This interpretation is supported by the fact that the hypercalcaemic rabbits excreted a greater percentage of the filtered sodium load than did the controls.

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