1. The effect of prolonged administration of β-adrenoreceptor-blocking agents was studied in growing rabbits. Equivalent doses of propranolol and timolol were given parenterally for 8 weeks.

2. Animals given propranolol had a significantly lower heart rate than that in the controls. After subcutaneous injection, plasma propranolol levels were twice those obtained after intraperitoneal injection.

3. Growth rate of animals given propranolol intraperitoneally was significantly less than controls, whereas animals given propranolol subcutaneously or timolol intraperitoneally grew normally. Reduced growth rate was associated with reduced food intake and was not related to the β-adrenoreceptor-blocking activity of the drugs.

4. At the end of the experiment the heart and other organs were examined. β-Adrenoreceptor blockade produced no significant changes in ventricular size or water content, and no significant changes in ultrastructure were found.

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