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Systemic vasodilation produced by sodium nitroprusside in various concentrations and accompanied by a decrease in baseline blood pressure was followed by progressive reduction in pressor responses to α-adrenoceptor agonist phenylephrine (mesatone) in rats. In a blood pressure range of below the physiological level (80-100 mm Hg), a positive linear correlation was revealed between the decrease in baseline blood pressure and pressor effect of phenylephrine.