The long-term constancy of the pressure-flow relationship of the upper urinary tract of the pig was studied in 8 pyeloureteral units and the short-term constancy in 10 units. No constancy was found for neither pelvic baseline pressure nor for pelvic perfusion pressure with an interval of 10 weeks. Pelvic pressure is a very unstable parameter with time but variating within the normal range. Contrarily, a good constancy was found for both baseline and perfusion pressure when 3 consecutive pressure-flow procedures performed with an interval of 30 min were compared to each other. It is concluded that the short-term constancy of the pressure-flow relationship may allow in vivo investigations of the function - if any - of drugs upon the pyeloureter.