Partial obstruction of the left ureter was created in newborn rats. Unobstruction was performed after 2 or 7 days. The investigations were carried out at 9 weeks of age--under slight hydropenia to institute an element of stress. Unobstruction was successful. On the unobstructed side, there were nevertheless impairments as compared to controls: urine osmolality (-32%), free water reabsorption (-44%), potassium excretion (-34%), renal blood flow (-36%) and glomerular filtration (-36%). On the intact contralateral side, tubular changes were the only signs of an attempt to compensate. Thus, consistent renal damage remained despite a very early unobstruction. Furthermore, the changes were similar to those we observed during long-term permanent obstruction. The injury seems to be established within a very short time and imitates the probable development seen in the affected fetus: after start of production, the urine is confronted by the preformed obstruction at the pyeloureteral junction. A high-pressure-prone system is built up and is not reversed, until the pelvis has become dilated and thus capable to buffer urinary flow peaks. Thereafter, no further deterioration occurs except in specific conditions. If clinically applicable, these observations implicate that there is no advantage with surgical intervention, even when performed early in fetal life, and there is no need for swift intervention, as the damage does not progress after its establishment. Most of the cases probably do not require surgery at all, unless pain, obvious functional impairment or urinary tract infection supervene.