The aim of this study was to investigate the role of nitric oxide (NO) in the regulation of blood flow in the porcine uterine artery during the course of the oestrous cycle. Experiments were carried out on animals on days 1-5, 8-13 and 17-20 of the oestrous cycle. After induction of anesthesia and opening of the abdominal cavity, blood samples were collected from the ovarian and uterine arteries and veins to determine nitrate and nitrite concentrations; on the opposite side to the blood sampling the branch of the uterine artery was prepared and a venous catheter was inserted into the artery. For measuring the changes in the blood pressure the catheter was connected via a polyvinyl cannula to a pressure transducer. Sodium nitroprusside (NP; 2.4 microg, 24 microg and 240 microg; a NO donor) or Nomega-nitro-L-arginine methyl ester (L-NAME; 2.2 microg, 22 microg, 220 microg and 2200 microg; an irreversible inhibitor of neuronal and endothelial NO synthase and reversible inhibitor of macrophage NO synthase) was administered via a bolus into the uterine artery. Nitrite/nitrate concentrations were: higher (P < 0.05) in the uterine vein as compared to the uterine artery on days 1-5 of the oestrous cycle; lower (P < 0.05) in the uterine artery as compared to the ovarian and uterine veins as well as in the ovarian artery as compared to the ovarian vein on days 8-13 of the cycle; lower (P < 0.05) in the uterine artery as compared to the ovarian artery and uterine and ovarian veins on days 17-20. Administration of NP at doses of 2.4 microg and 24 microg and L-NAME at all doses examined did not affect (P > 0.05) the blood pressure in the uterine artery in all periods examined. NP at a dose of 240 microg decreased (P < 0.001) the blood pressure in the arteries in all periods examined as compared to blood pressure before NP treatment. The results obtained indicate that NO is involved in the regulation of blood flow through the porcine reproductive tract. Moreover, our results suggest that the action of NP in the porcine uterine artery is not dependent on the phase of the oestrous cycle.