Renal excretion as seen in the urogram does not parallel changes in blood flow. The significance of the early urogram as a test of function is therefore very limited. 2. Following acute stenosis of one renal artery, there is simultaneous excretion by both kidneys until flow is reduced to 60%; exretion is delayed, with values of 0.5 to 7.5 minutes, following reduction to 30 to 60% of the original; there is no excretion below 30%. 3. Unilateral delay in excretion occurs at about the level of "critical occlusion pressure". 4. Delayed excretion is observed at the level of the so-called "critical stenosis" of the renal artery. A reduction of flow above 20% follows a reduction of the artery in excess of 66%, corresponding with a reduction of the lumen of more than 90%. 5. These findings indicate that delayed excretion in the early urogram is to be expected only as a result of severe stenosis of the renal artery and represents a late feature of renal vascular disease.