1. The role of baroreceptors in the cardiovascular mechanism of action of DL-propranolol has been studied by comparing the acute effects of subcutaneous injection of 1 and 5 mg/kg (3.3 x 10(-6) and 16.5 x 10(-6) mol/kg) of this drug in conscious baroreceptor-denervated spontaneously hypertensive (SH) rats with those in sham-operated control SH rats. 2. At 5 mg/kg (16.5 x 10(-6) mol/kg) propranolol caused a small, but significant, increase in blood pressure in sham-operated SH rats, whereas both after 1 and 5 mg/kg (3.3 x 10(-6) and 16.5 x 10(-6) mol/kg) immediate hypotension was observed in baroreceptor-denervated animals. 3. Heart rate dropped rapidly after injection of 1 or 5 mg/kg (3.3 x 10(-6) and 16.5 x 10(-6) mol/kg) propranolol both in the baroreceptor-denervated and sham-operated SH rats. Bradycardia was significantly larger in the baroreceptor-denervated animals after an injection of 5 mg/kg (16.5 x 10(-6) mol/kg). 4. It is concluded that the lack of an early hypotensive effect of propranolol in intact animals is caused by an increased baroreceptor reflex activity as a consequence of the fall in cardiac output.