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1. Methods for measuring the release of ⁴⁵Ca from isolated urinary bladders of toads (Bufo marinus) pre-loaded with this isotope have been devised. One method allowed separate collection from the mucosal and serosal surfaces of the bladders.

2. Reducing the ambient calcium concentration reduced the rate of ⁴⁵Ca efflux suggesting that efflux of radiolabel represents calcium exchange.

3. Antidiuretic hormone, theophylline and prostaglandin E₁ all increased calcium efflux, while lanthanum and amphotericin were without effect. Cyclic AMP caused only an inhibition of calcium release.

4. The increase in ⁴⁵Ca efflux due to antidiuretic hormone came exclusively from the mucosal side. Experiments with EGTA suggest that the calcium entering the mucosal solution arises mainly from superficial sites in the mucosal membrane.

5. The release of ⁴⁵Ca by hormone was not influenced by removal of sodium from the bathing solution. Low pH and amiloride reduced or abolished calcium release to hormone.

6. The time course of calcium release from the mucosal surface due to hormone was rapid (commencing between 0·5 and 1·5 min after hormone application). Thus calcium release precedes the increase in sodium transport and hydro-osmotic flow following hormone, and appears to be at least as rapid as cyclic AMP generation in the tissue.

7. The relationship between calcium release or exchange and the permeability changes in the bladder to water and to sodium, following hormone, are discussed.