In 7 piglets the pressure and peristaltic activity of both pyelo-ureters were investigated during low and high urine output and changes in bladder pressure and volume. Six ureters were shown to have spontaneous vesico-ureteric reflux, with a mean reflux producing bladder pressure of 26 cmH2O. During low urine output the median ureteric activity was of low frequency and identical in refluxing and patent ureters. When bladder filling was induced the median ureteric activity increased by 23% in the non-refluxing ureters and by 25% in refluxing ureters. During high urine output the median ureteric activity was slightly higher on the refluxing side. Under the influence of bladder filling the median ureteric activity increased with 131% in non-refluxing ureters and with 57% in refluxing ureters. In low urine output, the pelvic pressure was more or less independent of the bladder pressure, whereas during high urine output the pelvic pressure followed the bladder pressure in both refluxing and non-refluxing ureters. All peristaltic events were antegrade and complete. Thus no significant differences were found urodynamically between patent ureters and spontaneous refluxing, normally calibrated ureters.