Total umbilical cord occlusion and selective occlusion of the umbilical arteries and veins is associated with changes in uterine blood flow. In the present study, the resistance to uterine blood flow during selective occlusions of the umbilical arteries and/or veins was analysed in five chronically instrumented pregnant sheep in the last third part of pregnancy. An occluding device which allows separate occlusion of umbilical veins and arteries was applied to the umbilical cord. Median uterine artery blood flow was measured using an electromagnetic flow meter. Maternal pressures were measured in a branch of the uterine artery and vein. Two occlusions of the umbilical veins and/or arteries with a duration of 30-60 s were performed in each animal. Selective occlusion of the umbilical arteries resulted in a small increase in uterine blood flow from 637 +/- 79 ml/min during control toward 664 +/- 77 ml/min at the end of occlusion (p less than 0.05). Uterine perfusion pressure (uterine arterial pressure - uterine venous pressure) did not change. No changes were observed in calculated uterine vascular resistance (Poiseuille equation). Selective occlusion of the umbilical veins on the other hand caused a decrease in uterine blood flow from 617 +/- 75 ml/min during control to 546 +/- 69 ml/min at the end of occlusion (p less than 0.001), and a return to control value at 1 min after occlusion. The uterine perfusion pressure increased from 40.1 +/- 6.8 mmHg during control to 42.8 +/- 7.0 mmHg at the end of occlusion (p less than 0.01).(ABSTRACT TRUNCATED AT 250 WORDS)