The understanding of pathophysiology of obstructed uropathy has been facilitated by animal models with partial ureteric obstruction. Some studies on partially obstructed adult rats have drawn attention to a biphasic pattern of obstructive uropathy: an initial 'destructive' phase and a 'steady' phase in which renal deterioration no longer occurs and in which relief of obstruction would be of no advantage. We aimed to verify if this pattern applies also to younger (weanling) rats with more immature kidneys, resembling those of the human fetus. We measured the NAG-values in the urine samples of partially obstructed animals at different intervals of obstruction and in those of controls. The biphasic pattern proved to be the same as in adult rats as was previously documented, but the turning point occurred earlier (between 10 and 15 days of obstruction). Furthermore, there is evidence of low level values of N-Acetyl-Glucosaminidase (NAG) in the early phase of obstruction (5 days), demonstrating that the increase of tubular enzyme is not due to the operation itself. There is evidence that, if the 'destructive' phase can be precisely identified by biochemical studies, this could help identifying those subjects who could benefit from relief of obstruction.