The purpose of the present study was to determine the relative potency of nitroprusside and hydralazine with respect to inhibition of norepinephrine-induced contraction of isolated, uterine arteries from pregnant and nonpregnant patients. The arteries, obtained after hysterectomy, were dissected free from surrounding tissue, and arterial rings were prepared and mounted in tissue chambers filled with Kreb's-bicarbonate solution. Isometric tension was recorded. At concentrations of 10(-9) M to 10(-5) M, both nitroprusside and hydralazine produced concentration-dependent inhibition of the contractile response to norepinephrine. Nitroprusside and hydralazine were more potent in relaxing arteries contracted by a lower concentration (3 X 10(-6) M) of norepinephrine than by a higher concentration (10(-5) M) of norepinephrine. Regardless of the concentration of norepinephrine, nitroprusside was considerably more potent than hydralazine. The concentrations of nitroprusside that produced 50% inhibition (IC50) of the contractile response to norepinephrine (3 X 10(-6) M) in uterine arteries from pregnant and nonpregnant patients were 3.2 +/- 0.5 X 10(-9) M (n = 5) and 1.2 +/- 0.1 X 10(-9) M (n = 6), respectively. The IC50 values for hydralazine acting against norepinephrine (3 X 10(-6) M) in the uterine arteries from pregnant and nonpregnant patients were 5.1 +/- 0.5 X 10(-7) M (n = 5) and 4.0 +/- 0.5 X 10(-7) M (n = 6), respectively. Nitroprusside (10(-6) M), compared to hydralazine (10(-5) M), produced the greater maximal inhibition of norepinephrine-induced contraction.(ABSTRACT TRUNCATED AT 250 WORDS)