1. In vivo hyperosmolality was induced in two genera of amphibians (Bufo andRana) by dehydration or NaCl load, and maximal rate of rise of ventricular pressure and peak systolic pressure were measured with and without the conus arteriosus occluded. 2. Hyperosmolality and dehydration decreased maximal rate of rise of ventricular pressure. 3. Hyperosmolality and dehydration decreased resting and maximum ventricular systolic pressure. 4. The decline in ventricular muscle performance occurred despite a substantial degree of cell volume regulation in the ventricular muscle. 5. Potassium, sodium, and chloride contributed substantially to the increase in tissue solutes. 6. The negative inotropic effects of hyperosmolality and dehydration on amphibian cardiac muscle certainly contribute to the circulatory insufficiency associated with dehydration in amphibians.