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Abstract: The plant of Phyllanthus amarus is used as diuretic and to lower blood pressure in traditional medicine practice. The effect of the aqueous extract of the leaves of Phyllanthus amarus on blood pressure was evaluated in normotensive male rabbits. Intravenously administered aqueous doses (5 mg to 80 mg/kg) of the extract to anaesthesized normotensive male rabbits produced a significant fall in mean diastolic, systolic and mean arterial pressures in a graded dose response manner. The dose of 5 mg/kg produced the least hypotensive effect, causing a fall in mean diastolic, systolic, and mean arterial pressure of 13.3 ± 3.1, 19.7 ± 5.4, and14.3 ± 3.4 mmHg, respectively, while the dose of 80 mg/kg produced the greatest fall in mean diastolic, systolic, and mean arterial pressure of 49.7 ± 7.9, 45.5 ± 9.5, and 48.00 ± 6.5 mmHg, respectively. The extract had a greater blood pressure depressant effect on the diastolic blood pressure than on the systolic blood pressure. The highest dose of 80 mg/kg caused 62.5% fall in diastolic blood pressure, compared to the 33.2% fall in systolic blood pressure caused by the same dose. Atropine at the dose of 1 mg/kg blocked the hypotensive effect of the aqueous extract in a competitive manner. Promethazine at the dose of 1 mg/kg did not block the hypotensive effect of the aqueous extract, but potentiated the effect of the extract. After the administration of promethazine, the maximum tolerable dose of the extract was 40 mg/kg as compared to the initial dose of 80 mg/kg. The extract was found to decrease both the force and rate of myocardial contraction in a concentration dependent manner. The extract also dose dependently inhibited the intrinsic myogenic contraction of isolated rat portal vein. The results obtained show that the extract has blood pressure lowering effect which may be by the combined effects of myocardial depression, muscarinic receptor mediated vascular smooth muscle relaxation and by the calcium channel ion blockade in vascular smooth muscle