The aim of the present study was to investigate the influence of nitric oxide (NO) on the contractile activity of the isolated porcine ovarian and uterine arteries. Segments of the vessels, obtained from the pigs on days 1-5, 8-13 and 17-20 of the oestrous cycle, were mounted in the organ bath with Krebs-Ringer solution and contractile activity changes of the vessels were measured using isometric transducers. In Experiment I the arteries pretreated with norepinephrine (NE; 10(-7) M) were treated with sodium nitroprusside (SNP, 10(-8)-10(-4) M), a NO donor. In Experiment II administration of NE (10(-7) M) was preceded by treatment with Nomega-nitro-L-arginine methyl ester (L-NAME, 10(-8)-10(-6) M), an inhibitor of NO synthase. Donor of NO at doses of 10(-8)-10(-7) M did not affect (P>0.05) the contractility, while at doses of 10(-5)-10(-4) M caused a dose-dependent relaxation (P<0.05) of both ovarian and uterine arteries in all periods examined. Moreover, SNP at doses of 10(-6)-10(-4) M it caused significantly higher (P<0.05) relaxation of the ovarian arteries collected on days 8-13 as compared to the vessels from days 1-5 of the cycle. Pretreatment of the vessels with L-NAME caused a dose-dependent, significant (P<0.05) increase in the vasocontractile action of NE in both the ovarian and uterine arteries as compared to contractile activity of NE administered alone. Moreover, L-NAME pretreatment at a dose of 10(-6) M caused significantly higher (P<0.05) intensification of NE action in ovarian and uterine arteries collected on days 8-13 as compared to the vessels from days 1-5 (P<0.05) and 17-20 (P<0.05) of the oestrous cycle. Obtained results indicate that NO plays an important role in the regulation of the contractile activity of the isolated porcine ovarian and uterine arteries. Our data suggest that this action may be, at least in a part, dependent on the hormonal status of the organism.