1 day after bilateral ureteral occlusion (BUO), the extracellular fluid (ECF) volume of rats was increased by 2.8 ml/l00 g body weight above those values observed after either a sham operation or after 1 day of unilateral ureteral occlusion (UUO). The plasma urea concentration of BUO rats averaged about five times the values of either sham or UUO groups. The release of BUO of 1-day duration resulted in post-obstructive diuresis and natriuresis, whereas oliguria was observed after the release of UUO of 1-day duration. Contralateral nephrectomy of the normal kidney after the release of UUO 1-day duration (UUO-RN) resulted in an improvement of the damaged kidney functions, possibly due to an elevation of renal blood flow. Polyfructosan clearance of the UUO kidney was increased to a level significantly above the corresponding BUO value, 147 ± 34 as compared to 101 ± 8 μl/min/100 g body weight. However, post-obstructive diuresis was not observed. When the ECF volume of UUO-RN rats was elevated, towards those values observed in BUO rats by saline infusion, the rate of urine formation was insignificantly elevated above the nontreatment group, even though the sodium excretion rate was increased about three times the values of both the UUO-RN and BUO groups. When the plasma urea concentration of the UUO-RN group was elevated towards the BUO value, moderate diuresis was observed. But when both the ECF volume and plasma urea concentration of UUO-RN rats were increased towards the level of the BUO rats, post-obstructive natriuresis and diuresis were evident. Thus, the elevation of both the plasma urea concentration and the ECF volume contribute significantly to post-obstructive diuresis. However, the role of the natriuretic factors may not be eliminated due to the differences among the functioning nephrons between the BUO and UUO kidneys. The present studies were carried out to quantitate the contribution of ECF volume expansion and the elevation of plasma urea concentration on the induction of post-obstructive diuresis in rats.