Summary Renal function during and after release of a short period (less than 2 hours) and following release of a long period (6 hours) of total ureteric obstruction in the rat was studied in 2 series of experiments. Ureteric end pressures were found to be significantly higher in diuretic compared to dehydrated rats in the short period series of experiments. In both series of experiments nephron recovery following release of ureteric obstruction was greater in dehydrated compared to hydrated rats. The reasons for the difference of results in hydrated compared to dehydrated rats and sites of nephron damage are discussed. Possible relevance to clinical work is also propounded.