1. Oral and intravenous administration of metoprolol to adult spontaneously hypertensive rats (SHR) with established hypertension lowered arterial blood pressure within 4 days of treatment.
2. Steady-state plasma concentrations of metoprolol were similar to those of patients during antihypertensive treatment with this drug.
3. The neuroeffector function of portal veins of SHR treated orally for 14 days or intravenously for 4 days was not impaired when studied in vitro. This is in contrast to previous findings after long-term treatment.
4. It is concluded that the anti-hypertensive effect of metoprolol in SHR in many respects resembles that observed in patients. It is suggested that impairment of vasomotor nerve control may contribute to the anti-hypertensive effect of β-adrenoreceptor antagonists.