The effect on the blood pressure of experimental vesico-ureteric reflux was investigated in adult female Wistar rats. In 6 rats, reflux with isotonic saline produced a transient rise in systemic blood pressure followed by a fall, with return to normal within 2 min (mean BP readings: 121-130-93 mmHg). In 6 rats during water diuresis, reflux with distilled water produced similar changes (114-120-79 mmHg). In 6 rats in which the ureters were divided before reflux, no rise in blood pressure occurred although in 2 of these the pressure showed a marked fall. The pattern of blood pressure changes which occurred as a result of reflux was similar to that produced by a rapid i.v. injection of a corresponding volume of saline and it was concluded that the changes accompanying reflux are due to pyelovenous backflow. This was confirmed by producing reflux with 5% lissamine green which appeared in the peripheral capillaries within 4 s of the reflux in 3 rats. Slow dilatation of the pelvis with saline in 9 rats showed that rupture of the pelvic epithelium occurred at a mean pressure of 99 mmHg.